diff --git a/build.js b/build.js
index c6072a8..1ad5fbc 100644
--- a/build.js
+++ b/build.js
@@ -11,10 +11,11 @@ mkdirSync("dist", { recursive: true });
 
 // Build JavaScript
 await build({
-  entryPoints: ["src/game-viewport.js"],
+  entryPoints: ["src/index.js"],
   bundle: true,
+  splitting: true,
   format: "esm",
-  outfile: "dist/game-viewport.js",
+  outdir: "dist",
   sourcemap: true,
   platform: "browser",
 });
diff --git a/src/assets/data/classes/aether_sentinel.json b/src/assets/data/classes/aether_sentinel.json
new file mode 100644
index 0000000..eaa5e9f
--- /dev/null
+++ b/src/assets/data/classes/aether_sentinel.json
@@ -0,0 +1,38 @@
+{
+  "id": "CLASS_AETHER_SENTINEL",
+  "name": "Aether Sentinel",
+  "tier": 2,
+  "unlock_requirements": { "CLASS_CUSTODIAN": 5, "CLASS_VANGUARD": 3 },
+  "base_stats": {
+    "health": 130,
+    "attack": 8,
+    "defense": 9,
+    "magic": 6,
+    "speed": 5,
+    "willpower": 14,
+    "movement": 3
+  },
+  "growth_rates": {
+    "health": 11,
+    "defense": 1,
+    "willpower": 2
+  },
+  "starting_equipment": ["ITEM_GREATSHIELD", "ITEM_SANCTIFIED_MACE"],
+  "skillTreeData": {
+    "primary_stat": "willpower",
+    "secondary_stat": "defense",
+    "active_skills": [
+      "SKILL_GUARDIAN_LINK",
+      "SKILL_HOLY_NOVA",
+      "SKILL_VOW_SILENCE",
+      "SKILL_AETHER_CLEANSE",
+      "SKILL_DIVINE_INTERVENTION"
+    ],
+    "passive_skills": [
+      "PASSIVE_THORN_HEAL",
+      "PASSIVE_MARTYRDOM",
+      "PASSIVE_SACRED_GROUND",
+      "PASSIVE_UNSHAKEABLE"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/aether_weaver.json b/src/assets/data/classes/aether_weaver.json
new file mode 100644
index 0000000..3a852a7
--- /dev/null
+++ b/src/assets/data/classes/aether_weaver.json
@@ -0,0 +1,38 @@
+{
+  "id": "CLASS_WEAVER",
+  "name": "Aether Weaver",
+  "tier": 1,
+  "unlock_requirements": null,
+  "base_stats": {
+    "health": 80,
+    "attack": 5,
+    "defense": 4,
+    "magic": 12,
+    "speed": 10,
+    "willpower": 8,
+    "movement": 3
+  },
+  "growth_rates": {
+    "health": 6,
+    "magic": 2,
+    "speed": 1
+  },
+  "starting_equipment": ["ITEM_APPRENTICE_WAND", "ITEM_ROBES"],
+  "skillTreeData": {
+    "primary_stat": "magic",
+    "secondary_stat": "speed",
+    "active_skills": [
+      "SKILL_FIREBALL",
+      "SKILL_ICE_WALL",
+      "SKILL_TELEPORT",
+      "SKILL_CHAIN_LIGHTNING",
+      "SKILL_METEOR"
+    ],
+    "passive_skills": [
+      "PASSIVE_GLASS_CANNON",
+      "PASSIVE_MANA_SYPHON",
+      "PASSIVE_ELEMENTAL_AFFINITY",
+      "PASSIVE_DOUBLE_CAST"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/arcane_scourge.json b/src/assets/data/classes/arcane_scourge.json
new file mode 100644
index 0000000..5a0aa5c
--- /dev/null
+++ b/src/assets/data/classes/arcane_scourge.json
@@ -0,0 +1,38 @@
+{
+  "id": "CLASS_ARCANE_SCOURGE",
+  "name": "Arcane Scourge",
+  "tier": 2,
+  "unlock_requirements": { "CLASS_WEAVER": 5, "CLASS_SCAVENGER": 3 },
+  "base_stats": {
+    "health": 70,
+    "attack": 6,
+    "defense": 3,
+    "magic": 15,
+    "speed": 12,
+    "willpower": 6,
+    "movement": 4
+  },
+  "growth_rates": {
+    "health": 5,
+    "magic": 3,
+    "speed": 1
+  },
+  "starting_equipment": ["ITEM_VOID_STAFF", "ITEM_TATTERED_ROBES"],
+  "skillTreeData": {
+    "primary_stat": "magic",
+    "secondary_stat": "speed",
+    "active_skills": [
+      "SKILL_LIFE_TAP",
+      "SKILL_VOID_RAY",
+      "SKILL_SHARD_BLAST",
+      "SKILL_CHAOS_BOLT",
+      "SKILL_VOID_SINGULARITY"
+    ],
+    "passive_skills": [
+      "PASSIVE_RISK_TAKER",
+      "PASSIVE_SHARD_MAGNET",
+      "PASSIVE_OVERCHARGE",
+      "PASSIVE_DESPERATE_POWER"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/battle_mage.json b/src/assets/data/classes/battle_mage.json
new file mode 100644
index 0000000..7a3611a
--- /dev/null
+++ b/src/assets/data/classes/battle_mage.json
@@ -0,0 +1,39 @@
+{
+  "id": "CLASS_BATTLE_MAGE",
+  "name": "Battle Mage",
+  "tier": 2,
+  "unlock_requirements": { "CLASS_VANGUARD": 5, "CLASS_WEAVER": 3 },
+  "base_stats": {
+    "health": 110,
+    "attack": 10,
+    "defense": 7,
+    "magic": 10,
+    "speed": 9,
+    "willpower": 7,
+    "movement": 3
+  },
+  "growth_rates": {
+    "health": 9,
+    "attack": 1,
+    "magic": 1,
+    "defense": 1
+  },
+  "starting_equipment": ["ITEM_RUNE_BLADE", "ITEM_SPELL_SHIELD"],
+  "skillTreeData": {
+    "primary_stat": "attack",
+    "secondary_stat": "magic",
+    "active_skills": [
+      "SKILL_FLAME_STRIKE",
+      "SKILL_AEGIS",
+      "SKILL_WARP_STRIKE",
+      "SKILL_ARCANE_SLASH",
+      "SKILL_RUNE_STORM"
+    ],
+    "passive_skills": [
+      "PASSIVE_BATTLE_RHYTHM",
+      "PASSIVE_ARCANE_ARMOR",
+      "PASSIVE_SPELL_PARRY",
+      "PASSIVE_CONJURED_WEAPON"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/custodian.json b/src/assets/data/classes/custodian.json
new file mode 100644
index 0000000..49a127b
--- /dev/null
+++ b/src/assets/data/classes/custodian.json
@@ -0,0 +1,38 @@
+{
+  "id": "CLASS_CUSTODIAN",
+  "name": "Custodian",
+  "tier": 1,
+  "unlock_requirements": null,
+  "base_stats": {
+    "health": 95,
+    "attack": 6,
+    "defense": 5,
+    "magic": 8,
+    "speed": 6,
+    "willpower": 12,
+    "movement": 3
+  },
+  "growth_rates": {
+    "health": 8,
+    "willpower": 2,
+    "magic": 1
+  },
+  "starting_equipment": ["ITEM_STAFF", "ITEM_LEAF_ROBES"],
+  "skillTreeData": {
+    "primary_stat": "willpower",
+    "secondary_stat": "health",
+    "active_skills": [
+      "SKILL_MEND",
+      "SKILL_PURIFY",
+      "SKILL_SANCTUARY",
+      "SKILL_HASTE",
+      "SKILL_RESURRECT"
+    ],
+    "passive_skills": [
+      "PASSIVE_AURA_OF_PEACE",
+      "PASSIVE_FEEDBACK_LOOP",
+      "PASSIVE_OVERHEAL",
+      "PASSIVE_PACIFIST"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/field_engineer.json b/src/assets/data/classes/field_engineer.json
new file mode 100644
index 0000000..3a876f4
--- /dev/null
+++ b/src/assets/data/classes/field_engineer.json
@@ -0,0 +1,43 @@
+{
+  "id": "CLASS_FIELD_ENGINEER",
+  "name": "Field Engineer",
+  "tier": 2,
+  "unlock_requirements": { "CLASS_TINKER": 5, "CLASS_SCAVENGER": 3 },
+  "base_stats": {
+    "health": 105,
+    "attack": 9,
+    "defense": 6,
+    "magic": 0,
+    "speed": 10,
+    "willpower": 6,
+    "movement": 4,
+    "tech": 12
+  },
+  "growth_rates": {
+    "health": 8,
+    "speed": 1,
+    "tech": 2
+  },
+  "starting_equipment": [
+    "ITEM_AUTO_RIFLE",
+    "ITEM_REINFORCED_VEST",
+    "ITEM_REPAIR_DRONE"
+  ],
+  "skillTreeData": {
+    "primary_stat": "tech",
+    "secondary_stat": "speed",
+    "active_skills": [
+      "SKILL_SCRAP_CANNON",
+      "SKILL_PORTABLE_COVER",
+      "SKILL_REMOTE_OP",
+      "SKILL_EMP_BLAST",
+      "SKILL_ORBITAL_DROP"
+    ],
+    "passive_skills": [
+      "PASSIVE_COMBAT_SALVAGE",
+      "PASSIVE_FIELD_REFIT",
+      "PASSIVE_FAST_HANDS",
+      "PASSIVE_IMPROVISED_EXPLOSIVES"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/sapper.json b/src/assets/data/classes/sapper.json
new file mode 100644
index 0000000..bf09076
--- /dev/null
+++ b/src/assets/data/classes/sapper.json
@@ -0,0 +1,39 @@
+{
+  "id": "CLASS_SAPPER",
+  "name": "Sapper",
+  "tier": 2,
+  "unlock_requirements": { "CLASS_VANGUARD": 5, "CLASS_TINKER": 3 },
+  "base_stats": {
+    "health": 115,
+    "attack": 13,
+    "defense": 7,
+    "magic": 0,
+    "speed": 9,
+    "willpower": 5,
+    "movement": 4,
+    "tech": 8
+  },
+  "growth_rates": {
+    "health": 9,
+    "attack": 2,
+    "speed": 1
+  },
+  "starting_equipment": ["ITEM_EXPLOSIVE_HAMMER", "ITEM_BLAST_ARMOR"],
+  "skillTreeData": {
+    "primary_stat": "attack",
+    "secondary_stat": "tech",
+    "active_skills": [
+      "SKILL_BREACH_CHARGE",
+      "SKILL_ROCKET_JUMP",
+      "SKILL_TUNNEL_VISION",
+      "SKILL_CLUSTER_BOMB",
+      "SKILL_BIG_RED_BUTTON"
+    ],
+    "passive_skills": [
+      "PASSIVE_DEMOLITIONIST",
+      "PASSIVE_ANTI_COVER",
+      "PASSIVE_BLAST_SHIELD",
+      "PASSIVE_RECHARGE_BOMB"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/scavenger.json b/src/assets/data/classes/scavenger.json
new file mode 100644
index 0000000..9468ed4
--- /dev/null
+++ b/src/assets/data/classes/scavenger.json
@@ -0,0 +1,42 @@
+{
+  "id": "CLASS_SCAVENGER",
+  "name": "Scavenger",
+  "tier": 1,
+  "unlock_requirements": null,
+  "base_stats": {
+    "health": 90,
+    "attack": 8,
+    "defense": 5,
+    "magic": 2,
+    "speed": 12,
+    "willpower": 4,
+    "movement": 5
+  },
+  "growth_rates": {
+    "health": 7,
+    "speed": 2,
+    "attack": 1
+  },
+  "starting_equipment": [
+    "ITEM_DAGGER",
+    "ITEM_PADDED_VEST",
+    "ITEM_LOCKPICK_SET"
+  ],
+  "skillTreeData": {
+    "primary_stat": "speed",
+    "secondary_stat": "movement",
+    "active_skills": [
+      "SKILL_FLASHBANG",
+      "SKILL_GRAPPLE_HOOK",
+      "SKILL_STEALTH",
+      "SKILL_COIN_TOSS",
+      "SKILL_ASSASSINATE"
+    ],
+    "passive_skills": [
+      "PASSIVE_LUCKY_FIND",
+      "PASSIVE_BACKSTAB",
+      "PASSIVE_LIGHT_STEP",
+      "PASSIVE_HAGGLER"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/tinker.json b/src/assets/data/classes/tinker.json
new file mode 100644
index 0000000..7927c94
--- /dev/null
+++ b/src/assets/data/classes/tinker.json
@@ -0,0 +1,43 @@
+{
+  "id": "CLASS_TINKER",
+  "name": "Tinker",
+  "tier": 1,
+  "unlock_requirements": null,
+  "base_stats": {
+    "health": 100,
+    "attack": 10,
+    "defense": 6,
+    "magic": 0,
+    "speed": 7,
+    "willpower": 5,
+    "movement": 3,
+    "tech": 10
+  },
+  "growth_rates": {
+    "health": 8,
+    "attack": 1,
+    "tech": 2
+  },
+  "starting_equipment": [
+    "ITEM_WRENCH",
+    "ITEM_LEATHER_APRON",
+    "ITEM_TURRET_KIT"
+  ],
+  "skillTreeData": {
+    "primary_stat": "tech",
+    "secondary_stat": "defense",
+    "active_skills": [
+      "SKILL_DEPLOY_TURRET",
+      "SKILL_REPAIR_BOT",
+      "SKILL_OVERCLOCK",
+      "SKILL_SHOCK_GRENADE",
+      "SKILL_MECH_SUIT"
+    ],
+    "passive_skills": [
+      "PASSIVE_SCRAP_SHIELD",
+      "PASSIVE_EFFICIENT_BUILD",
+      "PASSIVE_RECYCLE",
+      "PASSIVE_CONDUCTIVE"
+    ]
+  }
+}
diff --git a/src/assets/data/classes/vanguard.json b/src/assets/data/classes/vanguard.json
new file mode 100644
index 0000000..36186cc
--- /dev/null
+++ b/src/assets/data/classes/vanguard.json
@@ -0,0 +1,38 @@
+{
+  "id": "CLASS_VANGUARD",
+  "name": "Vanguard",
+  "tier": 1,
+  "unlock_requirements": null,
+  "base_stats": {
+    "health": 120,
+    "attack": 12,
+    "defense": 8,
+    "magic": 0,
+    "speed": 8,
+    "willpower": 5,
+    "movement": 3
+  },
+  "growth_rates": {
+    "health": 10,
+    "attack": 1,
+    "defense": 1
+  },
+  "starting_equipment": ["ITEM_RUSTY_BLADE", "ITEM_SCRAP_PLATE"],
+  "skillTreeData": {
+    "primary_stat": "health",
+    "secondary_stat": "defense",
+    "active_skills": [
+      "SKILL_SHIELD_BASH",
+      "SKILL_TAUNT",
+      "SKILL_INTERCEPT",
+      "SKILL_EXECUTE",
+      "SKILL_AVATAR_OF_IRON"
+    ],
+    "passive_skills": [
+      "PASSIVE_IRON_SKIN",
+      "PASSIVE_THORNS",
+      "PASSIVE_UNYIELDING",
+      "PASSIVE_TITANS_GRIP"
+    ]
+  }
+}
diff --git a/src/factories/SkillTreeFactory.js b/src/factories/SkillTreeFactory.js
new file mode 100644
index 0000000..80d2beb
--- /dev/null
+++ b/src/factories/SkillTreeFactory.js
@@ -0,0 +1,103 @@
+/**
+ * SkillTreeFactory.js
+ * Generates class-specific skill trees by merging a Master Topology with Class Configuration.
+ */
+export class SkillTreeFactory {
+  /**
+   * @param {Object} templateRegistry - Map of Template IDs to JSON structures.
+   * @param {Object} skillRegistry - Map of Skill IDs to Skill Definitions.
+   */
+  constructor(templateRegistry, skillRegistry) {
+    this.templates = templateRegistry;
+    this.skills = skillRegistry;
+  }
+
+  /**
+   * Creates a fully hydrated Skill Tree for a specific class.
+   * @param {Object} classConfig - The Class definition containing 'skillTreeData'.
+   * @param {string} templateId - The ID of the topology to use (default: 'TEMPLATE_STANDARD_30').
+   */
+  createTree(classConfig, templateId = "TEMPLATE_STANDARD_30") {
+    const template = this.templates[templateId];
+    if (!template) throw new Error(`Template not found: ${templateId}`);
+
+    const config = classConfig.skillTreeData;
+    if (!config)
+      throw new Error(`Class ${classConfig.id} missing skillTreeData`);
+
+    const newTree = {
+      id: `TREE_${classConfig.id}`,
+      nodes: {},
+    };
+
+    // Iterate and Inject
+    for (const [nodeId, templateNode] of Object.entries(template.nodes)) {
+      // Clone the node structure to avoid mutating the template
+      let realNode = JSON.parse(JSON.stringify(templateNode));
+
+      // Hydrate based on Slot Type
+      this.hydrateNode(realNode, config, templateNode.tier);
+
+      newTree.nodes[nodeId] = realNode;
+    }
+
+    return newTree;
+  }
+
+  hydrateNode(node, config, tier) {
+    // Scaling Logic for Stats
+    const statValue = this.getTierStatValue(tier);
+
+    switch (node.type) {
+      case "SLOT_STAT_PRIMARY":
+        node.type = "STAT_BOOST";
+        node.data = {
+          stat: config.primary_stat,
+          value: statValue * 2, // Primary gets double value
+        };
+        break;
+
+      case "SLOT_STAT_SECONDARY":
+        node.type = "STAT_BOOST";
+        node.data = {
+          stat: config.secondary_stat,
+          value: statValue,
+        };
+        break;
+
+      case "SLOT_SKILL_ACTIVE_1":
+        node.type = "ACTIVE_SKILL";
+        // Map tier/slot to specific index in the config array
+        // Example: Slot 1 is the 0th skill
+        node.data = this.getSkillData(config.active_skills[0]);
+        break;
+
+      case "SLOT_SKILL_ACTIVE_2":
+        node.type = "ACTIVE_SKILL";
+        node.data = this.getSkillData(config.active_skills[1]);
+        break;
+
+      case "SLOT_SKILL_PASSIVE_1":
+        node.type = "PASSIVE_ABILITY";
+        node.data = { effect_id: config.passive_skills[0] };
+        break;
+
+      // ... Add cases for other slots (ULTIMATE, etc)
+
+      default:
+        // If it's already a concrete type (e.g. fixed layout), leave it alone
+        break;
+    }
+  }
+
+  getSkillData(skillId) {
+    const skill = this.skills[skillId];
+    if (!skill) return { id: skillId, name: "Unknown Skill" }; // Fallback
+    return skill;
+  }
+
+  getTierStatValue(tier) {
+    // Scaling logic: Tier 1 = 1, Tier 5 = 5
+    return tier;
+  }
+}
diff --git a/src/index.html b/src/index.html
index 928f84b..dd70910 100644
--- a/src/index.html
+++ b/src/index.html
@@ -12,7 +12,7 @@
       href="https://fonts.googleapis.com/css2?family=Chakra+Petch:wght@400;700&family=Cinzel:wght@700&display=swap"
       rel="stylesheet"
     />
-
+    <script type="module" src="index.js"></script>
     <style>
       :root {
         /* Palette Definition */
@@ -341,61 +341,10 @@
       </div>
     </div>
 
+    <team-builder class="hidden" aria-label="Team Builder"></team-builder>
     <!-- GAME VIEWPORT CONTAINER -->
     <game-viewport class="hidden" aria-label="Game World"></game-viewport>
 
     <!-- GAME LOGIC (MODULE SCRIPT) -->
-    <script type="module">
-      // --- 2. Accessibility Helper ---
-      function announce(message) {
-        const announcer = document.getElementById("a11y-announcer");
-        announcer.textContent = message;
-      }
-
-      // --- 3. New Descent Logic (Using Dynamic Import) ---
-      // We attach listener inside the module script because module scope is local
-      document
-        .getElementById("btn-start")
-        .addEventListener("click", startNewDescent);
-
-      async function startNewDescent() {
-        // A. Update State & UI
-        const landingUI = document.getElementById("landing-ui");
-        const loader = document.getElementById("loading-overlay");
-        const loadingMsg = document.getElementById("loading-message");
-
-        landingUI.classList.add("hidden");
-        loader.classList.remove("hidden");
-
-        // B. Accessibility Updates
-        announce("Starting new game. Loading game engine.");
-        loadingMsg.textContent = "LOADING GAME COMPONENT...";
-
-        // C. Lazy Load logic (Components registered via import above)
-        try {
-          // Simulate loading time
-          setTimeout(() => {
-            loadingMsg.textContent = "GENERATING VOXEL GRID...";
-            initializeGameWorld();
-          }, 1000);
-        } catch (error) {
-          console.error("Failed to load game:", error);
-          loadingMsg.textContent = "ERROR LOADING ENGINE. PLEASE REFRESH.";
-          announce("Error loading game engine. Please refresh.");
-        }
-      }
-
-      // --- 4. Game Initialization ---
-      async function initializeGameWorld() {
-        const loader = document.getElementById("loading-overlay");
-        const gameViewport = document.querySelector("game-viewport");
-        await import("./game-viewport.js");
-
-        // D. Transition to Game
-        loader.classList.add("hidden");
-        gameViewport.classList.remove("hidden");
-        announce("Game loaded. Tactical grid active.");
-      }
-    </script>
   </body>
 </html>
diff --git a/src/index.js b/src/index.js
new file mode 100644
index 0000000..9620f7c
--- /dev/null
+++ b/src/index.js
@@ -0,0 +1,63 @@
+const loader = document.getElementById("loading-overlay");
+const landingUI = document.getElementById("landing-ui");
+const loadingMsg = document.getElementById("loading-message");
+
+// --- 2. Accessibility Helper ---
+function announce(message) {
+  const announcer = document.getElementById("a11y-announcer");
+  announcer.textContent = message;
+}
+
+// --- 3. New Descent Logic (Using Dynamic Import) ---
+// We attach listener inside the module script because module scope is local
+document.getElementById("btn-start").addEventListener("click", startNewDescent);
+
+async function startNewDescent() {
+  landingUI.classList.add("hidden");
+  loader.classList.remove("hidden");
+
+  // B. Accessibility Updates
+  announce("Starting new game. Entering the Team Builder.");
+  loadingMsg.textContent = "LOADING TEAM BUILDER COMPONENT...";
+
+  // C. Lazy Load logic (Components registered via import above)
+  try {
+    initiateTeamBuilder();
+  } catch (error) {
+    console.error("Failed to load team builder:", error);
+    loadingMsg.textContent = "ERROR LOADING TEAM BUILDER. PLEASE REFRESH.";
+    announce("Error loading team builder. Please refresh.");
+  }
+  // try {
+  //   // Simulate loading time
+  //   setTimeout(() => {
+  //     loadingMsg.textContent = "GENERATING VOXEL GRID...";
+  //     initializeGameWorld();
+  //   }, 1000);
+  // } catch (error) {
+  //   console.error("Failed to load game:", error);
+  //   loadingMsg.textContent = "ERROR LOADING ENGINE. PLEASE REFRESH.";
+  //   announce("Error loading game engine. Please refresh.");
+  // }
+}
+
+async function initiateTeamBuilder() {
+  await import("./ui/team-builder.js");
+  const teamBuilder = document.querySelector("team-builder");
+  document.startViewTransition(() => {
+    teamBuilder.classList.remove("hidden");
+    loader.classList.add("hidden");
+  });
+  announce("Team Builder loaded. Ready to build your team.");
+}
+
+// --- 4. Game Initialization ---
+async function initializeGameWorld() {
+  const gameViewport = document.querySelector("game-viewport");
+  await import("./ui/game-viewport.js");
+
+  // D. Transition to Game
+  loader.classList.add("hidden");
+  gameViewport.classList.remove("hidden");
+  announce("Game loaded. Tactical grid active.");
+}
diff --git a/src/items/Item.js b/src/items/Item.js
new file mode 100644
index 0000000..d2c10f6
--- /dev/null
+++ b/src/items/Item.js
@@ -0,0 +1,54 @@
+/**
+ * Item.js
+ * Represents a piece of equipment or consumable.
+ */
+export class Item {
+  /**
+   * @param {Object} def - The JSON definition of the item.
+   */
+  constructor(def) {
+    this.id = def.id;
+    this.name = def.name;
+    this.type = def.type; // WEAPON, ARMOR, UTILITY, RELIC
+    this.rarity = def.rarity || "COMMON";
+    this.tags = def.tags || [];
+
+    // Base Stats (e.g. { attack: 5, defense: 2 })
+    this.stats = def.stats || {};
+
+    // Passive Effects (Event Listeners)
+    // e.g. { trigger: "ON_HIT", action: "APPLY_STATUS", ... }
+    this.passives = def.passive_effects || [];
+
+    // Active Ability (Grants a new action button)
+    // e.g. { ability_id: "SKILL_FIREBALL" }
+    this.activeAbility = def.active_ability || null;
+
+    // Requirements to equip
+    this.requirements = def.requirements || {};
+  }
+
+  /**
+   * Returns true if the unit meets the requirements to equip this.
+   */
+  canEquip(unit) {
+    if (this.requirements.class_lock) {
+      // Check if unit's active class is in the allowed list
+      if (!this.requirements.class_lock.includes(unit.activeClassId)) {
+        return false;
+      }
+    }
+
+    if (this.requirements.min_stat) {
+      for (const [stat, value] of Object.entries(this.requirements.min_stat)) {
+        if (unit.baseStats[stat] < value) return false;
+      }
+    }
+
+    return true;
+  }
+
+  getStat(statName) {
+    return this.stats[statName] || 0;
+  }
+}
diff --git a/src/items/tier1_gear.json b/src/items/tier1_gear.json
new file mode 100644
index 0000000..130b754
--- /dev/null
+++ b/src/items/tier1_gear.json
@@ -0,0 +1,117 @@
+[
+  {
+    "id": "ITEM_RUSTY_BLADE",
+    "name": "Rusty Infantry Blade",
+    "type": "WEAPON",
+    "rarity": "COMMON",
+    "tags": ["PHYSICAL", "MELEE"],
+    "stats": { "attack": 3 },
+    "description": "Standard issue jagged metal. Reliable but heavy."
+  },
+  {
+    "id": "ITEM_SCRAP_PLATE",
+    "name": "Scrap Plate Armor",
+    "type": "ARMOR",
+    "rarity": "COMMON",
+    "tags": ["HEAVY"],
+    "stats": { "defense": 3, "speed": -1 },
+    "description": "Cobbled together from ruin debris."
+  },
+  {
+    "id": "ITEM_APPRENTICE_WAND",
+    "name": "Apprentice Spark-Wand",
+    "type": "WEAPON",
+    "rarity": "COMMON",
+    "tags": ["MAGIC", "RANGED"],
+    "stats": { "magic": 4 },
+    "description": "A cracked crystal on a stick. Leaks sparks."
+  },
+  {
+    "id": "ITEM_ROBES",
+    "name": "Novice Robes",
+    "type": "ARMOR",
+    "rarity": "COMMON",
+    "tags": ["LIGHT"],
+    "stats": { "willpower": 3, "magic": 1 },
+    "description": "Standard issue for the Arcane Dominion."
+  },
+  {
+    "id": "ITEM_DAGGER",
+    "name": "Scavenger's Shiv",
+    "type": "WEAPON",
+    "rarity": "COMMON",
+    "tags": ["PHYSICAL", "MELEE", "LIGHT"],
+    "stats": { "attack": 2, "speed": 1 },
+    "description": "Sharp, rusty, and easy to hide."
+  },
+  {
+    "id": "ITEM_PADDED_VEST",
+    "name": "Padded Vest",
+    "type": "ARMOR",
+    "rarity": "COMMON",
+    "tags": ["LIGHT"],
+    "stats": { "defense": 1, "movement": 1 },
+    "description": "Doesn't stop a sword, but helps with the cold."
+  },
+  {
+    "id": "ITEM_LOCKPICK_SET",
+    "name": "Lockpick Set",
+    "type": "UTILITY",
+    "rarity": "COMMON",
+    "tags": ["TECH"],
+    "stats": { "tech": 2 },
+    "passive_effects": [
+      {
+        "trigger": "ON_INTERACT",
+        "condition": "TARGET_LOCKED",
+        "action": "UNLOCK",
+        "chance": 0.5
+      }
+    ]
+  },
+  {
+    "id": "ITEM_WRENCH",
+    "name": "Heavy Wrench",
+    "type": "WEAPON",
+    "rarity": "COMMON",
+    "tags": ["PHYSICAL", "TECH", "BLUNT"],
+    "stats": { "attack": 4, "tech": 2 },
+    "description": "Good for fixing turrets and breaking knees."
+  },
+  {
+    "id": "ITEM_LEATHER_APRON",
+    "name": "Smith's Apron",
+    "type": "ARMOR",
+    "rarity": "COMMON",
+    "tags": ["MEDIUM"],
+    "stats": { "defense": 2, "fire_resist": 5 },
+    "description": "Thick leather to protect against sparks."
+  },
+  {
+    "id": "ITEM_TURRET_KIT",
+    "name": "Turret Construction Kit",
+    "type": "UTILITY",
+    "rarity": "COMMON",
+    "tags": ["TECH"],
+    "stats": { "tech": 3 },
+    "active_ability": { "ability_id": "SKILL_DEPLOY_TURRET" }
+  },
+  {
+    "id": "ITEM_STAFF",
+    "name": "Quarterstaff",
+    "type": "WEAPON",
+    "rarity": "COMMON",
+    "tags": ["PHYSICAL", "MAGIC", "BLUNT"],
+    "stats": { "attack": 3, "willpower": 2 },
+    "description": "Simple wood, balanced for defense."
+  },
+  {
+    "id": "ITEM_LEAF_ROBES",
+    "name": "Sanctuary Robes",
+    "type": "ARMOR",
+    "rarity": "COMMON",
+    "tags": ["LIGHT", "ORGANIC"],
+    "stats": { "willpower": 4, "health": 10 },
+    "description": "Woven from living vines."
+  }
+]
diff --git a/src/game-viewport.js b/src/ui/game-viewport.js
similarity index 91%
rename from src/game-viewport.js
rename to src/ui/game-viewport.js
index 6deebab..5a9e1ee 100644
--- a/src/game-viewport.js
+++ b/src/ui/game-viewport.js
@@ -1,8 +1,8 @@
 import { LitElement, html, css } from "lit";
 import * as THREE from "three";
 import { OrbitControls } from "three/addons/controls/OrbitControls.js";
-import { VoxelGrid } from "./grid/VoxelGrid.js";
-import { VoxelManager } from "./grid/VoxelManager.js";
+import { VoxelGrid } from "../grid/VoxelGrid.js";
+import { VoxelManager } from "../grid/VoxelManager.js";
 
 export class GameViewport extends LitElement {
   static styles = css`
@@ -75,10 +75,10 @@ export class GameViewport extends LitElement {
     // 1. Create Data Grid
     this.voxelGrid = new VoxelGrid(20, 8, 20);
 
-    const { CaveGenerator } = await import("./generation/CaveGenerator.js");
-    const { RuinGenerator } = await import("./generation/RuinGenerator.js");
+    const { CaveGenerator } = await import("../generation/CaveGenerator.js");
+    const { RuinGenerator } = await import("../generation/RuinGenerator.js");
     const { CrystalSpiresGenerator } = await import(
-      "./generation/CrystalSpiresGenerator.js"
+      "../generation/CrystalSpiresGenerator.js"
     );
     const crystalSpiresGen = new CrystalSpiresGenerator(this.voxelGrid, 12345);
     crystalSpiresGen.generate(5, 8);
diff --git a/src/ui/team-builder.js b/src/ui/team-builder.js
new file mode 100644
index 0000000..5e267f9
--- /dev/null
+++ b/src/ui/team-builder.js
@@ -0,0 +1,320 @@
+import { LitElement, html, css } from "lit";
+
+export class TeamBuilder extends LitElement {
+  static get styles() {
+    return css`
+      :host {
+        display: block;
+        position: absolute;
+        top: 0;
+        left: 0;
+        width: 100%;
+        height: 100%;
+        font-family: "Courier New", monospace; /* Placeholder for Voxel Font */
+        color: white;
+        pointer-events: none; /* Let clicks pass through to 3D scene where empty */
+        z-index: 10;
+      }
+
+      .container {
+        display: grid;
+        grid-template-columns: 250px 1fr 250px;
+        grid-template-rows: 1fr 80px;
+        height: 100%;
+        width: 100%;
+        pointer-events: auto;
+        background: rgba(0, 0, 0, 0.4); /* Dim background */
+      }
+
+      /* --- LEFT PANEL: ROSTER --- */
+      .roster-panel {
+        background: rgba(20, 20, 30, 0.9);
+        border-right: 2px solid #555;
+        padding: 1rem;
+        overflow-y: auto;
+      }
+
+      .class-card {
+        background: #333;
+        border: 2px solid #555;
+        padding: 10px;
+        margin-bottom: 10px;
+        cursor: pointer;
+        transition: all 0.2s;
+        display: flex;
+        align-items: center;
+        gap: 10px;
+      }
+
+      .class-card:hover {
+        border-color: #00ffff;
+        background: #444;
+      }
+
+      .class-card.locked {
+        opacity: 0.5;
+        pointer-events: none;
+        filter: grayscale(1);
+      }
+
+      /* --- CENTER PANEL: SLOTS --- */
+      .squad-panel {
+        display: flex;
+        justify-content: center;
+        align-items: flex-end;
+        padding-bottom: 2rem;
+        gap: 20px;
+      }
+
+      .squad-slot {
+        width: 120px;
+        height: 150px;
+        background: rgba(0, 0, 0, 0.6);
+        border: 2px dashed #666;
+        display: flex;
+        flex-direction: column;
+        align-items: center;
+        justify-content: center;
+        cursor: pointer;
+        position: relative;
+      }
+
+      .squad-slot.filled {
+        border: 2px solid #00ff00;
+        background: rgba(0, 50, 0, 0.6);
+      }
+
+      .squad-slot.selected {
+        border-color: #00ffff;
+        box-shadow: 0 0 10px #00ffff;
+      }
+
+      .remove-btn {
+        position: absolute;
+        top: -10px;
+        right: -10px;
+        background: red;
+        border: none;
+        color: white;
+        width: 24px;
+        height: 24px;
+        border-radius: 50%;
+        cursor: pointer;
+        font-weight: bold;
+      }
+
+      /* --- RIGHT PANEL: DETAILS --- */
+      .details-panel {
+        background: rgba(20, 20, 30, 0.9);
+        border-left: 2px solid #555;
+        padding: 1rem;
+      }
+
+      /* --- FOOTER --- */
+      .footer {
+        grid-column: 1 / -1;
+        display: flex;
+        justify-content: center;
+        align-items: center;
+        background: rgba(10, 10, 20, 0.95);
+        border-top: 2px solid #555;
+      }
+
+      .embark-btn {
+        padding: 15px 40px;
+        font-size: 1.5rem;
+        background: #008800;
+        color: white;
+        border: 2px solid #00ff00;
+        cursor: pointer;
+        text-transform: uppercase;
+        font-weight: bold;
+      }
+
+      .embark-btn:disabled {
+        background: #333;
+        border-color: #555;
+        color: #777;
+        cursor: not-allowed;
+      }
+    `;
+  }
+
+  static get properties() {
+    return {
+      availableClasses: { type: Array }, // Input: List of class definition objects
+      squad: { type: Array }, // Internal State: The 4 slots
+      selectedSlotIndex: { type: Number },
+      hoveredClass: { type: Object },
+    };
+  }
+
+  constructor() {
+    super();
+    this.squad = [null, null, null, null];
+    this.selectedSlotIndex = 0; // Default to first slot
+    this.availableClasses = []; // Passed in by parent
+    this.hoveredClass = null;
+  }
+
+  render() {
+    const isSquadValid = this.squad.some((u) => u !== null);
+
+    return html`
+      <div class="container">
+        <!-- ROSTER LIST -->
+        <div class="roster-panel">
+          <h3>Roster</h3>
+          ${this.availableClasses.map(
+            (cls) => html`
+              <div
+                class="class-card ${cls.unlocked ? "" : "locked"}"
+                @click="${() => this._assignClass(cls)}"
+                @mouseenter="${() => (this.hoveredClass = cls)}"
+                @mouseleave="${() => (this.hoveredClass = null)}"
+              >
+                <div class="icon">${cls.icon || "⚔️"}</div>
+                <div>
+                  <strong>${cls.name}</strong><br />
+                  <small>${cls.role}</small>
+                </div>
+              </div>
+            `
+          )}
+        </div>
+
+        <!-- CENTER SQUAD SLOTS -->
+        <div class="squad-panel">
+          ${this.squad.map(
+            (unit, index) => html`
+              <div
+                class="squad-slot ${unit ? "filled" : ""} ${this
+                  .selectedSlotIndex === index
+                  ? "selected"
+                  : ""}"
+                @click="${() => this._selectSlot(index)}"
+              >
+                ${unit
+                  ? html`
+                      <div class="icon" style="font-size: 2rem;">
+                        ${unit.icon || "🛡️"}
+                      </div>
+                      <span>${unit.name}</span>
+                      <button
+                        class="remove-btn"
+                        @click="${(e) => this._removeUnit(e, index)}"
+                      >
+                        X
+                      </button>
+                    `
+                  : html`<span
+                      >Slot ${index + 1}<br /><small>Select Class</small></span
+                    >`}
+              </div>
+            `
+          )}
+        </div>
+
+        <!-- RIGHT DETAILS PANEL -->
+        <div class="details-panel">
+          ${this.hoveredClass
+            ? html`
+                <h2>${this.hoveredClass.name}</h2>
+                <p><em>${this.hoveredClass.role}</em></p>
+                <hr />
+                <p>
+                  ${this.hoveredClass.description ||
+                  "No description available."}
+                </p>
+                <h4>Base Stats</h4>
+                <ul>
+                  <li>HP: ${this.hoveredClass.base_stats?.health}</li>
+                  <li>AP: ${this.hoveredClass.base_stats?.speed}</li>
+                  <!-- Simplified AP calc -->
+                  <li>Move: ${this.hoveredClass.base_stats?.movement}</li>
+                </ul>
+              `
+            : html`<p>Hover over a class to see details.</p>`}
+        </div>
+
+        <!-- FOOTER -->
+        <div class="footer">
+          <button
+            class="embark-btn"
+            ?disabled="${!isSquadValid}"
+            @click="${this._handleEmbark}"
+          >
+            DESCEND
+          </button>
+        </div>
+      </div>
+    `;
+  }
+
+  // --- LOGIC ---
+
+  _selectSlot(index) {
+    this.selectedSlotIndex = index;
+  }
+
+  _assignClass(classDef) {
+    if (!classDef.unlocked) return;
+
+    // 1. Create a lightweight manifest for the slot
+    const unitManifest = {
+      classId: classDef.id,
+      name: classDef.name, // In real app, auto-generate name
+      icon: classDef.icon,
+    };
+
+    // 2. Update State (Trigger Re-render)
+    const newSquad = [...this.squad];
+    newSquad[this.selectedSlotIndex] = unitManifest;
+    this.squad = newSquad;
+
+    // 3. Auto-advance selection
+    if (this.selectedSlotIndex < 3) {
+      this.selectedSlotIndex++;
+    }
+
+    // 4. Dispatch Event (For 3D Scene to show model)
+    this.dispatchEvent(
+      new CustomEvent("squad-update", {
+        detail: { slot: this.selectedSlotIndex, unit: unitManifest },
+        bubbles: true,
+        composed: true,
+      })
+    );
+  }
+
+  _removeUnit(e, index) {
+    e.stopPropagation(); // Prevent slot selection
+    const newSquad = [...this.squad];
+    newSquad[index] = null;
+    this.squad = newSquad;
+    this.selectedSlotIndex = index; // Select the empty slot
+
+    // Dispatch Event (To clear 3D model)
+    this.dispatchEvent(
+      new CustomEvent("squad-update", {
+        detail: { slot: index, unit: null },
+        bubbles: true,
+        composed: true,
+      })
+    );
+  }
+
+  _handleEmbark() {
+    const manifest = this.squad.filter((u) => u !== null);
+
+    this.dispatchEvent(
+      new CustomEvent("embark", {
+        detail: { squad: manifest },
+        bubbles: true,
+        composed: true,
+      })
+    );
+  }
+}
+
+customElements.define("team-builder", TeamBuilder);
diff --git a/src/units/Explorer.js b/src/units/Explorer.js
new file mode 100644
index 0000000..2d70e51
--- /dev/null
+++ b/src/units/Explorer.js
@@ -0,0 +1,114 @@
+import { Unit } from "./Unit.js";
+
+/**
+ * Explorer.js
+ * Player character class supporting Multi-Class Mastery and Persistent Progression.
+ */
+export class Explorer extends Unit {
+  constructor(id, name, startingClassId, classDefinition) {
+    super(id, name, "EXPLORER", `${startingClassId}_MODEL`);
+
+    this.activeClassId = startingClassId;
+
+    // Persistent Mastery: Tracks progress for EVERY class this character has played
+    // Key: ClassID, Value: { level, xp, skillPoints, unlockedNodes[] }
+    this.classMastery = {};
+
+    // Initialize the starting class entry
+    this.initializeMastery(startingClassId);
+
+    // Hydrate stats based on the provided definition
+    if (classDefinition) {
+      this.recalculateBaseStats(classDefinition);
+      this.currentHealth = this.baseStats.health;
+    }
+
+    // Inventory
+    this.equipment = {
+      weapon: null,
+      armor: null,
+      utility: null,
+      relic: null,
+    };
+
+    // Active Skills (Populated by Skill Tree)
+    this.actions = [];
+    this.passives = [];
+  }
+
+  initializeMastery(classId) {
+    if (!this.classMastery[classId]) {
+      this.classMastery[classId] = {
+        level: 1,
+        xp: 0,
+        skillPoints: 0,
+        unlockedNodes: [],
+      };
+    }
+  }
+
+  /**
+   * Updates base stats based on the active class's base + growth rates * level.
+   * @param {Object} classDef - The JSON definition of the class stats.
+   */
+  recalculateBaseStats(classDef) {
+    if (classDef.id !== this.activeClassId) {
+      console.warn(
+        `Mismatch: Recalculating stats for ${this.activeClassId} using definition for ${classDef.id}`
+      );
+    }
+
+    const mastery = this.classMastery[this.activeClassId];
+
+    // 1. Start with Class Defaults
+    let stats = { ...classDef.base_stats };
+
+    // 2. Add Level Growth
+    // (Level 1 is base, so growth applies for levels 2+)
+    const levelsGained = mastery.level - 1;
+    if (levelsGained > 0) {
+      for (let stat in classDef.growth_rates) {
+        if (stats[stat] !== undefined) {
+          stats[stat] += classDef.growth_rates[stat] * levelsGained;
+        }
+      }
+    }
+
+    this.baseStats = stats;
+    this.maxHealth = stats.health; // Update MaxHP cap
+  }
+
+  /**
+   * Swaps the active class logic.
+   * NOTE: Does NOT check unlock requirements (handled by UI/MetaSystem).
+   */
+  changeClass(newClassId, newClassDef) {
+    // 1. Ensure mastery record exists
+    this.initializeMastery(newClassId);
+
+    // 2. Switch ID
+    this.activeClassId = newClassId;
+
+    // 3. Update Model ID (Visuals)
+    this.voxelModelID = `${newClassId}_MODEL`;
+
+    // 4. Recalculate Stats for the new job
+    this.recalculateBaseStats(newClassDef);
+
+    // 5. Reset Current HP to new Max (or keep percentage? Standard is reset)
+    this.currentHealth = this.baseStats.health;
+  }
+
+  /**
+   * Adds XP to the *current* class.
+   */
+  gainExperience(amount) {
+    const mastery = this.classMastery[this.activeClassId];
+    mastery.xp += amount;
+    // Level up logic would be handled by a system checking XP curves
+  }
+
+  getLevel() {
+    return this.classMastery[this.activeClassId].level;
+  }
+}
diff --git a/src/units/Unit.js b/src/units/Unit.js
new file mode 100644
index 0000000..560f3eb
--- /dev/null
+++ b/src/units/Unit.js
@@ -0,0 +1,60 @@
+/**
+ * Unit.js
+ * Base class for all entities on the grid (Explorers, Enemies, Structures).
+ */
+export class Unit {
+  constructor(id, name, type, voxelModelID) {
+    this.id = id;
+    this.name = name;
+    this.type = type; // 'EXPLORER', 'ENEMY', 'STRUCTURE'
+    this.voxelModelID = voxelModelID;
+
+    // Grid State
+    this.position = { x: 0, y: 0, z: 0 };
+    this.facing = "NORTH";
+
+    // Combat State
+    this.currentHealth = 100;
+    this.maxHealth = 100; // Derived from effective stats later
+
+    this.currentAP = 0; // Action Points for current turn
+    this.chargeMeter = 0; // Dynamic Initiative (0-100)
+
+    this.statusEffects = []; // Active debuffs/buffs
+
+    // Base Stats (Raw values before gear/buffs)
+    this.baseStats = {
+      health: 100,
+      attack: 10,
+      defense: 5,
+      magic: 0,
+      speed: 10,
+      willpower: 5,
+      movement: 4,
+      tech: 0,
+    };
+  }
+
+  /**
+   * Updates position.
+   * Note: Validation happens in VoxelGrid/MovementSystem.
+   */
+  setPosition(x, y, z) {
+    this.position = { x, y, z };
+  }
+
+  /**
+   * Consumes AP. Returns true if successful.
+   */
+  spendAP(amount) {
+    if (this.currentAP >= amount) {
+      this.currentAP -= amount;
+      return true;
+    }
+    return false;
+  }
+
+  isAlive() {
+    return this.currentHealth > 0;
+  }
+}
diff --git a/test/factories/SkillTreeFactory.test.js b/test/factories/SkillTreeFactory.test.js
new file mode 100644
index 0000000..286fe1a
--- /dev/null
+++ b/test/factories/SkillTreeFactory.test.js
@@ -0,0 +1,68 @@
+import { expect } from "@esm-bundle/chai";
+import { SkillTreeFactory } from "../../src/factories/SkillTreeFactory.js";
+
+describe("System: Skill Tree Factory", () => {
+  let factory;
+  let mockTemplates;
+  let mockSkills;
+  let mockClassConfig;
+
+  beforeEach(() => {
+    // 1. Setup Mock Data
+    mockTemplates = {
+      TEMPLATE_STANDARD_30: {
+        nodes: {
+          ROOT_NODE: {
+            tier: 1,
+            type: "SLOT_STAT_PRIMARY",
+            children: ["CHILD_NODE"],
+          },
+          CHILD_NODE: { tier: 1, type: "SLOT_SKILL_ACTIVE_1", children: [] },
+        },
+      },
+    };
+
+    mockSkills = {
+      SKILL_FIREBALL: { id: "SKILL_FIREBALL", name: "Fireball", damage: 10 },
+    };
+
+    mockClassConfig = {
+      id: "TEST_CLASS",
+      skillTreeData: {
+        primary_stat: "attack",
+        secondary_stat: "defense",
+        active_skills: ["SKILL_FIREBALL"],
+        passive_skills: ["PASSIVE_BURNING"],
+      },
+    };
+
+    // 2. Initialize Factory
+    factory = new SkillTreeFactory(mockTemplates, mockSkills);
+  });
+
+  it("CoA 1: Should maintain the topology (structure) of the template", () => {
+    const tree = factory.createTree(mockClassConfig);
+
+    expect(tree.nodes).to.have.property("ROOT_NODE");
+    expect(tree.nodes).to.have.property("CHILD_NODE");
+    expect(tree.nodes["ROOT_NODE"].children).to.include("CHILD_NODE");
+  });
+
+  it("CoA 2: Should inject Primary Stats based on Class Config", () => {
+    const tree = factory.createTree(mockClassConfig);
+    const rootNode = tree.nodes["ROOT_NODE"];
+
+    expect(rootNode.type).to.equal("STAT_BOOST");
+    expect(rootNode.data.stat).to.equal("attack"); // Injected from config
+    expect(rootNode.data.value).to.equal(2); // Tier 1 * 2
+  });
+
+  it("CoA 3: Should inject Active Skills based on Class Config", () => {
+    const tree = factory.createTree(mockClassConfig);
+    const childNode = tree.nodes["CHILD_NODE"];
+
+    expect(childNode.type).to.equal("ACTIVE_SKILL");
+    // Should resolve the full skill object from registry
+    expect(childNode.data.name).to.equal("Fireball");
+  });
+});
diff --git a/test/items/Item.test.js b/test/items/Item.test.js
new file mode 100644
index 0000000..d9b48d6
--- /dev/null
+++ b/test/items/Item.test.js
@@ -0,0 +1,73 @@
+import { expect } from "@esm-bundle/chai";
+import { Item } from "../../src/items/Item.js";
+
+// Mock Unit for Requirement Checking
+const mockVanguard = {
+  activeClassId: "CLASS_VANGUARD",
+  baseStats: { attack: 10, tech: 0 },
+};
+
+const mockTinker = {
+  activeClassId: "CLASS_TINKER",
+  baseStats: { attack: 5, tech: 10 },
+};
+
+describe("System: Items", () => {
+  let swordDef;
+  let techGunDef;
+
+  beforeEach(() => {
+    swordDef = {
+      id: "ITEM_TEST_SWORD",
+      name: "Test Sword",
+      type: "WEAPON",
+      stats: { attack: 5 },
+      requirements: { min_stat: { attack: 8 } },
+    };
+
+    techGunDef = {
+      id: "ITEM_TEST_GUN",
+      type: "WEAPON",
+      stats: { attack: 8 },
+      requirements: { class_lock: ["CLASS_TINKER"] },
+    };
+  });
+
+  it("CoA 1: Should store basic stats correctly", () => {
+    const item = new Item(swordDef);
+    expect(item.getStat("attack")).to.equal(5);
+    expect(item.getStat("magic")).to.equal(0); // Undefined stat
+  });
+
+  it("CoA 2: Should enforce Min Stat requirements", () => {
+    const item = new Item(swordDef);
+
+    // Vanguard has 10 Atk (Passes > 8)
+    expect(item.canEquip(mockVanguard)).to.be.true;
+
+    // Tinker has 5 Atk (Fails < 8)
+    expect(item.canEquip(mockTinker)).to.be.false;
+  });
+
+  it("CoA 3: Should enforce Class Lock requirements", () => {
+    const item = new Item(techGunDef);
+
+    // Vanguard cannot equip Tinker items
+    expect(item.canEquip(mockVanguard)).to.be.false;
+
+    // Tinker can
+    expect(item.canEquip(mockTinker)).to.be.true;
+  });
+
+  it("CoA 4: Should handle Active Ability grants", () => {
+    const kitDef = {
+      id: "ITEM_KIT",
+      type: "UTILITY",
+      active_ability: { ability_id: "SKILL_DEPLOY" },
+    };
+    const item = new Item(kitDef);
+
+    expect(item.activeAbility).to.exist;
+    expect(item.activeAbility.ability_id).to.equal("SKILL_DEPLOY");
+  });
+});
diff --git a/test/units/Explorer.test.js b/test/units/Explorer.test.js
new file mode 100644
index 0000000..dedab9d
--- /dev/null
+++ b/test/units/Explorer.test.js
@@ -0,0 +1,73 @@
+import { expect } from "@esm-bundle/chai";
+import { Explorer } from "../../src/units/Explorer.js";
+
+// Mock Class Definitions
+const CLASS_VANGUARD = {
+  id: "CLASS_VANGUARD",
+  base_stats: { health: 100, attack: 10, speed: 5 },
+  growth_rates: { health: 10, attack: 1 },
+};
+
+const CLASS_TINKER = {
+  id: "CLASS_TINKER",
+  base_stats: { health: 80, attack: 8, speed: 7 },
+  growth_rates: { health: 5, attack: 2 },
+};
+
+describe("Unit: Explorer Class Logic", () => {
+  it("CoA 1: Should initialize with base stats from definition", () => {
+    const hero = new Explorer("p1", "Hero", "CLASS_VANGUARD", CLASS_VANGUARD);
+
+    expect(hero.baseStats.health).to.equal(100);
+    expect(hero.baseStats.attack).to.equal(10);
+    expect(hero.classMastery["CLASS_VANGUARD"]).to.exist;
+    expect(hero.classMastery["CLASS_VANGUARD"].level).to.equal(1);
+  });
+
+  it("CoA 2: Should calculate stats based on Level Growth", () => {
+    const hero = new Explorer("p1", "Hero", "CLASS_VANGUARD", CLASS_VANGUARD);
+
+    // Manually level up to 3
+    hero.classMastery["CLASS_VANGUARD"].level = 3;
+    hero.recalculateBaseStats(CLASS_VANGUARD);
+
+    // Level 3 means 2 level-ups.
+    // Health: 100 + (10 * 2) = 120
+    // Attack: 10 + (1 * 2) = 12
+    expect(hero.baseStats.health).to.equal(120);
+    expect(hero.baseStats.attack).to.equal(12);
+  });
+
+  it("CoA 3: changeClass should switch stats and persist old progress", () => {
+    const hero = new Explorer("p1", "Hero", "CLASS_VANGUARD", CLASS_VANGUARD);
+
+    // Level up Vanguard
+    hero.classMastery["CLASS_VANGUARD"].level = 5;
+    hero.recalculateBaseStats(CLASS_VANGUARD);
+    expect(hero.baseStats.health).to.equal(140); // 100 + 40
+
+    // Switch to Tinker (New Job)
+    hero.changeClass("CLASS_TINKER", CLASS_TINKER);
+
+    // Should have Level 1 Tinker Stats
+    expect(hero.activeClassId).to.equal("CLASS_TINKER");
+    expect(hero.baseStats.health).to.equal(80); // Base Tinker
+
+    // Verify Vanguard history is saved
+    expect(hero.classMastery["CLASS_VANGUARD"].level).to.equal(5);
+  });
+
+  it("CoA 4: Switching BACK to old class should restore high stats", () => {
+    const hero = new Explorer("p1", "Hero", "CLASS_VANGUARD", CLASS_VANGUARD);
+    hero.classMastery["CLASS_VANGUARD"].level = 5;
+
+    // Switch Away
+    hero.changeClass("CLASS_TINKER", CLASS_TINKER);
+
+    // Switch Back
+    hero.changeClass("CLASS_VANGUARD", CLASS_VANGUARD);
+
+    // Should be back to Level 5 Stats
+    expect(hero.baseStats.health).to.equal(140);
+  });
+});
diff --git a/test/units/Unit.test.js b/test/units/Unit.test.js
new file mode 100644
index 0000000..e69de29