diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 00000000..ce625dfa
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,68 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Lint
+        run: npm run lint
+
+  type-check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Type check
+        run: npm run type-check
+
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Test
+        run: npm run test
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - name: Install dependencies
+        run: npm ci
+      - name: Build
+        run: npm run build
diff --git a/.gitignore b/.gitignore
index 136a9d6b..5ef861ec 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@
 
 # Testing
 /coverage
+/dist-tests
 
 # Next.js
 /.next/
diff --git a/README.md b/README.md
index 928e7485..2066318a 100644
--- a/README.md
+++ b/README.md
@@ -360,6 +360,7 @@ Areas where contributions would be valuable:
 ```bash
 npm run type-check  # TypeScript validation
 npm run lint        # ESLint
+npm run test        # Unit tests
 npm run dev         # Development server
 ```
 
diff --git a/package.json b/package.json
index 5a711bad..7d9cf89b 100644
--- a/package.json
+++ b/package.json
@@ -9,6 +9,7 @@
     "start": "next start",
     "lint": "next lint",
     "type-check": "tsc --noEmit",
+    "test": "rm -rf dist-tests && tsc -p tsconfig.test.json && node --require ./scripts/test-alias.cjs --test $(find dist-tests/tests -name \"*.test.js\")",
     "optimize-models": "node scripts/optimize-models.js",
     "generate-manifests": "node scripts/generate-music-manifest.js"
   },
diff --git a/scripts/test-alias.cjs b/scripts/test-alias.cjs
new file mode 100644
index 00000000..fa1a7d1c
--- /dev/null
+++ b/scripts/test-alias.cjs
@@ -0,0 +1,19 @@
+const path = require('path');
+const Module = require('module');
+
+const originalResolveFilename = Module._resolveFilename;
+
+Module._resolveFilename = function resolveFilename(request, parent, isMain, options) {
+  if (request.startsWith('@/')) {
+    const resolvedPath = path.resolve(
+      __dirname,
+      '..',
+      'dist-tests',
+      'src',
+      request.slice(2)
+    );
+    return originalResolveFilename.call(this, resolvedPath, parent, isMain, options);
+  }
+
+  return originalResolveFilename.call(this, request, parent, isMain, options);
+};
diff --git a/tests/engine/core/gameLoop.test.ts b/tests/engine/core/gameLoop.test.ts
new file mode 100644
index 00000000..9f519715
--- /dev/null
+++ b/tests/engine/core/gameLoop.test.ts
@@ -0,0 +1,47 @@
+import { GameLoop } from '@/engine/core/GameLoop';
+import assert from 'node:assert/strict';
+import { afterEach, describe, it } from 'node:test';
+
+const wait = (ms: number): Promise<void> =>
+  new Promise((resolve) => setTimeout(resolve, ms));
+
+describe('GameLoop fallback timing', () => {
+  const originalWorker = globalThis.Worker;
+
+  afterEach(() => {
+    globalThis.Worker = originalWorker;
+  });
+
+  it('ticks at the configured rate using the fallback interval', async () => {
+    globalThis.Worker = undefined as unknown as typeof Worker;
+
+    const updates: number[] = [];
+    const loop = new GameLoop(20, (delta) => updates.push(delta));
+
+    loop.start();
+    await wait(220);
+    loop.stop();
+
+    assert.ok(updates.length >= 3);
+    assert.ok(updates.length <= 6);
+    assert.ok(updates.every((delta) => delta === 50));
+  });
+
+  it('respects tick rate changes while running', async () => {
+    globalThis.Worker = undefined as unknown as typeof Worker;
+
+    const updates: number[] = [];
+    const loop = new GameLoop(10, (delta) => updates.push(delta));
+
+    loop.start();
+    await wait(240);
+    const beforeChange = updates.length;
+    loop.setTickRate(5);
+    await wait(450);
+    loop.stop();
+
+    assert.ok(beforeChange >= 1);
+    assert.ok(updates.length > beforeChange);
+    assert.ok(updates.slice(beforeChange).every((delta) => delta === 200));
+  });
+});
diff --git a/tests/engine/ecs/entityId.test.ts b/tests/engine/ecs/entityId.test.ts
new file mode 100644
index 00000000..ddaa2701
--- /dev/null
+++ b/tests/engine/ecs/entityId.test.ts
@@ -0,0 +1,78 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import {
+  EntityIdAllocator,
+  INVALID_ENTITY_ID,
+  MAX_ENTITY_INDEX,
+  MAX_GENERATION,
+  getEntityGeneration,
+  getEntityIndex,
+  isInvalidEntityId,
+  packEntityId,
+} from '@/engine/ecs/EntityId';
+
+describe('EntityId utilities', () => {
+  it('packs and unpacks entity ids', () => {
+    const id = packEntityId(42, 7);
+    assert.strictEqual(getEntityIndex(id), 42);
+    assert.strictEqual(getEntityGeneration(id), 7);
+  });
+
+  it('reserves invalid entity id sentinel', () => {
+    assert.ok(isInvalidEntityId(INVALID_ENTITY_ID));
+    assert.strictEqual(getEntityIndex(INVALID_ENTITY_ID), 0);
+    assert.strictEqual(getEntityGeneration(INVALID_ENTITY_ID), 0);
+  });
+
+  it('masks indices and generations to allowed ranges', () => {
+    const id = packEntityId(MAX_ENTITY_INDEX + 10, MAX_GENERATION + 5);
+    assert.strictEqual(getEntityIndex(id), MAX_ENTITY_INDEX + 10 & MAX_ENTITY_INDEX);
+    assert.strictEqual(getEntityGeneration(id), (MAX_GENERATION + 5) & MAX_GENERATION);
+  });
+});
+
+describe('EntityIdAllocator', () => {
+  it('allocates and validates ids, then recycles with generation increment', () => {
+    const allocator = new EntityIdAllocator(4);
+
+    const first = allocator.allocate();
+    const second = allocator.allocate();
+
+    assert.ok(allocator.isValid(first));
+    assert.ok(allocator.isValid(second));
+    assert.strictEqual(allocator.getAllocatedCount(), 2);
+
+    allocator.free(first);
+
+    assert.strictEqual(allocator.isValid(first), false);
+    assert.strictEqual(allocator.getFreeCount(), 1);
+
+    const recycled = allocator.allocate();
+    assert.strictEqual(getEntityIndex(recycled), getEntityIndex(first));
+    assert.ok(getEntityGeneration(recycled) > getEntityGeneration(first));
+  });
+
+  it('returns invalid id when capacity exceeded', () => {
+    const allocator = new EntityIdAllocator(2);
+
+    const first = allocator.allocate();
+    const second = allocator.allocate();
+    const third = allocator.allocate();
+
+    assert.ok(!isInvalidEntityId(first));
+    assert.ok(isInvalidEntityId(second));
+    assert.ok(isInvalidEntityId(third));
+  });
+
+  it('resets allocator state on clear', () => {
+    const allocator = new EntityIdAllocator(8);
+    allocator.allocate();
+    allocator.allocate();
+
+    allocator.clear();
+
+    assert.strictEqual(allocator.getAllocatedCount(), 0);
+    assert.strictEqual(allocator.getFreeCount(), 0);
+    assert.strictEqual(allocator.getStats().highWaterMark, 0);
+  });
+});
diff --git a/tests/engine/network/merkleTree.test.ts b/tests/engine/network/merkleTree.test.ts
new file mode 100644
index 00000000..40a4f93c
--- /dev/null
+++ b/tests/engine/network/merkleTree.test.ts
@@ -0,0 +1,88 @@
+import {
+  MerkleTreeBuilder,
+  MerkleTreeComparator,
+  type MerkleTreeData,
+} from '@/engine/network/MerkleTree';
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+const buildTree = (hashOverrides?: { unit2?: number }): MerkleTreeData => {
+  const unit1 = MerkleTreeBuilder.createEntityNode(1, 'unit', 111);
+  const unit2 = MerkleTreeBuilder.createEntityNode(2, 'unit', hashOverrides?.unit2 ?? 222);
+
+  const playerGroup = MerkleTreeBuilder.createGroupNode('player1', [unit2, unit1]);
+  const unitsCategory = MerkleTreeBuilder.createCategoryNode('units', [playerGroup]);
+  const root = MerkleTreeBuilder.createRootNode([unitsCategory]);
+
+  return {
+    root,
+    tick: 10,
+    timestamp: 1234,
+    entityCount: 2,
+  };
+};
+
+describe('MerkleTree utilities', () => {
+  it('builds deterministic hashes regardless of child ordering', () => {
+    const treeA = buildTree();
+
+    const unit1 = MerkleTreeBuilder.createEntityNode(1, 'unit', 111);
+    const unit2 = MerkleTreeBuilder.createEntityNode(2, 'unit', 222);
+    const playerGroup = MerkleTreeBuilder.createGroupNode('player1', [unit1, unit2]);
+    const unitsCategory = MerkleTreeBuilder.createCategoryNode('units', [playerGroup]);
+    const root = MerkleTreeBuilder.createRootNode([unitsCategory]);
+
+    const treeB: MerkleTreeData = {
+      root,
+      tick: 10,
+      timestamp: 1234,
+      entityCount: 2,
+    };
+
+    assert.strictEqual(treeA.root.hash, treeB.root.hash);
+  });
+
+  it('identifies divergent entities', () => {
+    const local = buildTree();
+    const remote = buildTree({ unit2: 999 });
+
+    const result = MerkleTreeComparator.findDivergence(local.root, remote.root);
+
+    assert.ok(result.entityIds.includes(2));
+    assert.strictEqual(result.path[0], 'root');
+    assert.ok(result.comparisons > 0);
+  });
+
+  it('finds divergent categories and groups from network summaries', () => {
+    const tree = buildTree();
+    const serialized = MerkleTreeComparator.serializeForNetwork(tree);
+
+    assert.strictEqual(MerkleTreeComparator.isIdentical(tree, serialized), true);
+
+    const tampered = {
+      ...serialized,
+      categoryHashes: {
+        ...serialized.categoryHashes,
+        units: serialized.categoryHashes.units + 1,
+      },
+    };
+
+    assert.deepStrictEqual(MerkleTreeComparator.findDivergentCategories(tree, tampered), ['units']);
+
+    const tamperedGroups = {
+      ...serialized,
+      groupHashes: {
+        ...serialized.groupHashes,
+        units: {
+          ...serialized.groupHashes.units,
+          player1: serialized.groupHashes.units.player1 + 1,
+        },
+      },
+    };
+
+    assert.deepStrictEqual(
+      MerkleTreeComparator.findDivergentGroups(tree, tamperedGroups, 'units'),
+      ['player1']
+    );
+  });
+});
diff --git a/tests/utils/fixedPoint.test.ts b/tests/utils/fixedPoint.test.ts
new file mode 100644
index 00000000..2ac79f3a
--- /dev/null
+++ b/tests/utils/fixedPoint.test.ts
@@ -0,0 +1,60 @@
+import {
+  FP_MAX_SAFE,
+  FP_MIN_SAFE,
+  deterministicDamage,
+  deterministicDistance,
+  fpDiv,
+  fpFromFloat,
+  fpMul,
+  fpToFloat,
+  integerSqrt,
+  quantize,
+  dequantize,
+} from '@/utils/FixedPoint';
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+describe('FixedPoint utilities', () => {
+  it('round-trips floats through fixed-point conversion', () => {
+    const value = 1.2345;
+    const fixed = fpFromFloat(value);
+    const roundTrip = fpToFloat(fixed);
+
+    assert.ok(Math.abs(roundTrip - value) < 0.0002);
+  });
+
+  it('multiplies fixed-point values deterministically', () => {
+    const a = fpFromFloat(1.5);
+    const b = fpFromFloat(-2.25);
+    const result = fpMul(a, b);
+
+    assert.ok(Math.abs(fpToFloat(result) + 3.375) < 0.0002);
+  });
+
+  it('divides with zero denominator safely', () => {
+    assert.strictEqual(fpDiv(10, 0), FP_MAX_SAFE);
+    assert.strictEqual(fpDiv(-10, 0), FP_MIN_SAFE);
+  });
+
+  it('computes integer square roots deterministically', () => {
+    assert.strictEqual(integerSqrt(0), 0);
+    assert.strictEqual(integerSqrt(1), 1);
+    assert.strictEqual(integerSqrt(15), 3);
+    assert.strictEqual(integerSqrt(16), 4);
+    assert.strictEqual(integerSqrt(0x80000000), 46340);
+  });
+
+  it('computes deterministic distance on quantized coordinates', () => {
+    assert.ok(Math.abs(deterministicDistance(0, 0, 3, 4) - 5) < 0.0001);
+  });
+
+  it('enforces minimum deterministic damage', () => {
+    const damage = deterministicDamage(5, 1, 10);
+    assert.strictEqual(damage, 1);
+  });
+
+  it('quantizes and dequantizes values consistently', () => {
+    const quantized = quantize(3.14159, 1000);
+    assert.ok(Math.abs(dequantize(quantized, 1000) - 3.142) < 0.001);
+  });
+});
diff --git a/tsconfig.test.json b/tsconfig.test.json
new file mode 100644
index 00000000..a290febf
--- /dev/null
+++ b/tsconfig.test.json
@@ -0,0 +1,23 @@
+{
+  "compilerOptions": {
+    "target": "ES2019",
+    "module": "commonjs",
+    "moduleResolution": "node",
+    "lib": ["ES2019", "DOM"],
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "noEmit": false,
+    "outDir": "dist-tests",
+    "baseUrl": ".",
+    "paths": {
+      "@/*": ["src/*"]
+    },
+    "jsx": "react-jsx",
+    "resolveJsonModule": true,
+    "types": ["node", "react"]
+  },
+  "include": ["tests/**/*.test.ts"],
+  "exclude": ["node_modules", "dist-tests"]
+}