var playerGold = 30;
var playerScore = 0;
var Tower = Container.expand(function (gameInstance) {
	var self = Container.call(this);
	self.game = gameInstance;
	var towerGraphics = self.createAsset('towerSprite', 'Tower Sprite', .5, .5);
	towerGraphics.scale.set(2);
	self.shoot = function (enemies) {
		if (self.shootCounter++ % 60 === 0) {
			for (var i = 0; i < enemies.length; i++) {
				var enemy = enemies[i];
				var dx = enemy.x - self.x;
				var dy = enemy.y - self.y;
				var distance = Math.sqrt(dx * dx + dy * dy);
				if (distance <= 300) {
					console.log('shot');
					var bullet = new Bullet();
					bullet.x = self.x;
					bullet.y = self.y;
					bullet.target = enemy;
					var tx = bullet.target.x - bullet.x;
					var ty = bullet.target.y - bullet.y;
					var rad = Math.atan2(ty, tx);
					bullet.rotation = rad + Math.PI / 2;
					bullet.vx = Math.cos(rad);
					bullet.vy = Math.sin(rad);
					self.game.addBullet(bullet);
					break;
				}
			}
		}
	};
	self.upgrade = function () {};
	self.shootCounter = 0;
	self.health = 100;
});
var Enemy = Container.expand(function () {
	var self = Container.call(this);
	var enemyGraphics = self.createAsset('enemySprite', 'Enemy Sprite', .5, .5);
	enemyGraphics.scale.set(0.8);
	self.hitpoints = 3;
	self.zigzagTension = Math.floor(Math.random() * 40) + 10;
	self.move = function (towers) {
		var minDist = Infinity;
		var closestTower = null;
		for (var i = 0; i < self.game.towers.length; i++) {
			var dx = self.game.towers[i].x - self.x;
			var dy = self.game.towers[i].y - self.y;
			var dist = Math.sqrt(dx * dx + dy * dy);
			if (dist < minDist) {
				minDist = dist;
				closestTower = self.game.towers[i];
			}
		}
		if (closestTower && minDist < closestTower.width / 2 + this.width / 2) {
			closestTower.health -= 1;
			if (closestTower.health <= 0) {
				closestTower.destroy();
			}
		} else if (closestTower && minDist < 100) {
			var avoidanceForce = 8;
			var ax = this.x - closestTower.x;
			var ay = this.y - closestTower.y;
			var len = Math.sqrt(ax * ax + ay * ay);
			ax = ax / len * avoidanceForce;
			ay = ay / len * avoidanceForce;
			this.x += ax;
			this.y += ay;
		} else {
			this.zigzagCounter = this.zigzagCounter || 0;
			this.zigzagDirection = this.zigzagDirection || 1;
			if (this.zigzagCounter++ % self.zigzagTension === 0) this.zigzagDirection *= -1;
			this.x += 4 * this.zigzagDirection;
			this.y += 2;
			if (Math.random() < 0.01) {
				this.zigzagTension = Math.floor(Math.random() * 40) + 10;
			}
			var scaleIncrement = (1 - 0.1) / 200;
			this.scale.x += scaleIncrement;
			this.scale.y += scaleIncrement;
			this.scale.x = Math.min(this.scale.x, 1);
			this.scale.y = Math.min(this.scale.y, 1);
		}
	};
	self.die = function () {
		playerGold += 5;
		this.game.updateGoldLabel();
		var index = this.game.enemies.indexOf(this);
		if (index > -1) {
			this.game.enemies.splice(index, 1);
		}
		this.destroy();
	};
});
var Bullet = Container.expand(function () {
	var self = Container.call(this);
	self.game = null;
	var bulletGraphics = self.createAsset('bullet', 'Bullet Graphics', .5, .5);
	bulletGraphics.scale.set(0.2);
	self.vx = 0;
	self.vy = 0;
	self.move = function () {
		if (this.target) {
			var tx = this.target.x - this.x;
			var ty = this.target.y - this.y;
			var dist = Math.sqrt(tx * tx + ty * ty);
			if (dist > 0) {
				this.x += this.vx * 10;
				this.y += this.vy * 10;
			}
			if (dist < this.width / 2 + this.target.width / 2) {
				this.hit();
			} else if (this.x < 0 || this.x > 2048 || this.y < 0 || this.y > 2732) {
				this.destroy();
			}
		}
	};
	self.hit = function () {
		this.target.hitpoints -= 1;
		if (this.target.hitpoints <= 0) {
			this.target.die();
		}
		this.destroy();
	};
});
var Game = Container.expand(function () {
	var self = Container.call(this);
	self.updateGoldLabel = function () {
		goldLabel.setText('Gold: ' + playerGold);
	};
	self.playerGold = 0;
	self.addBullet = function (bullet) {
		bullets.push(bullet);
		self.addChild(bullet);
	};
	var background = self.createAsset('background', 'Background Image', 0, 0);
	background.width = 2048;
	background.height = 2732;
	background.y = 0;
	self.addChildAt(background, 0);
	self.towers = [];
	self.enemies = [];
	var bullets = [];
	var selectedTowerType = null;
	var initialEnemy = new Enemy();
	initialEnemy.game = self;
	initialEnemy.x = 300 + Math.random() * (1848 - 300);
	initialEnemy.y = 200;
	initialEnemy.scale.set(0.1);
	self.enemies.push(initialEnemy);
	self.addChild(initialEnemy);
	LK.on('tick', function () {
		self.enemySpawnCounter = self.enemySpawnCounter || 0;
		if (++self.enemySpawnCounter >= 120) {
			var newEnemy = new Enemy();
			newEnemy.game = self;
			newEnemy.x = 300 + Math.random() * (1848 - 300);
			newEnemy.y = 200;
			newEnemy.scale.set(0.1);
			self.enemies.push(newEnemy);
			self.addChild(newEnemy);
			self.enemySpawnCounter = 0;
		}
		for (var i = 0; i < self.enemies.length; i++) {
			self.enemies[i].move();
			if (self.enemies[i].y >= 2732 - self.enemies[i].height / 2) {
				LK.showGameOver();
			}
		}
		for (var j = 0; j < self.towers.length; j++) {
			self.towers[j].shoot(self.enemies);
		}
		for (var k = bullets.length - 1; k >= 0; k--) {
			bullets[k].move();
			if (!bullets[k].parent) {
				bullets.splice(k, 1);
			}
		}
	});
	stage.on('down', function (obj) {
		var position = obj.event.getLocalPosition(self);
		if (position.y >= 400 && position.y <= 2250) {
			if (playerGold >= 10) {
				playerGold -= 10;
				var newTower = self.addChild(new Tower(self));
				newTower.x = position.x;
				newTower.y = position.y;
				self.towers.push(newTower);
				self.updateGoldLabel();
			}
		}
	});
	stage.on('move', function (obj) {});
	stage.on('up', function (obj) {});
	var bottomBlackBox = self.createAsset('blackBox', 'Black Box at bottom', 0, 1);
	bottomBlackBox.width = 2048;
	bottomBlackBox.height = 400;
	bottomBlackBox.y = 2732;
	self.addChild(bottomBlackBox);
	var goldLabel = new Text2('Gold: 30', {
		size: 100,
		fill: '#ffd700',
		align: 'left'
	});
	goldLabel.x = 120;
	goldLabel.y = 2632 - bottomBlackBox.height + (bottomBlackBox.height - goldLabel.height) / 2;
	self.addChild(goldLabel);
	var scoreLabel = new Text2('Score: 0', {
		size: 100,
		fill: '#ccbbff',
		align: 'left'
	});
	scoreLabel.x = 120;
	scoreLabel.y = 2772 - bottomBlackBox.height + (bottomBlackBox.height - scoreLabel.height) / 2;
	self.addChild(scoreLabel);
});

===================================================================
--- original.js
+++ change.js
@@ -1,8 +1,9 @@
 var playerGold = 30;
 var playerScore = 0;
-var Tower = Container.expand(function () {
+var Tower = Container.expand(function (gameInstance) {
 	var self = Container.call(this);
+	self.game = gameInstance;
 	var towerGraphics = self.createAsset('towerSprite', 'Tower Sprite', .5, .5);
 	towerGraphics.scale.set(2);
 	self.shoot = function (enemies) {
 		if (self.shootCounter++ % 60 === 0) {
@@ -22,9 +23,9 @@
 					var rad = Math.atan2(ty, tx);
 					bullet.rotation = rad + Math.PI / 2;
 					bullet.vx = Math.cos(rad);
 					bullet.vy = Math.sin(rad);
-					self.parent.addBullet(bullet);
+					self.game.addBullet(bullet);
 					break;
 				}
 			}
 		}
@@ -180,9 +181,9 @@
 		var position = obj.event.getLocalPosition(self);
 		if (position.y >= 400 && position.y <= 2250) {
 			if (playerGold >= 10) {
 				playerGold -= 10;
-				var newTower = self.addChild(new Tower());
+				var newTower = self.addChild(new Tower(self));
 				newTower.x = position.x;
 				newTower.y = position.y;
 				self.towers.push(newTower);
 				self.updateGoldLabel();