/**** 
* Classes
****/
// Define the Hero class
var Hero = Container.expand(function () {
	var self = Container.call(this);
	var heroGraphics = self.createAsset('soccerPlayer', 'Hero character', 0.5, 0.5);
	self.speed = 5;
	self.shootInterval = 30;
	self.shootTick = 0;
	self.moveLeft = function () {
		self.x = Math.max(self.width / 2, self.x - self.speed * 4);
	};
	self.moveRight = function () {
		self.x = Math.min(2048 - self.width / 2, self.x + self.speed * 4);
	};
	self.shoot = function () {
		if (self.shootTick <= 0) {
			var bullet = new HeroBullet();
			bullet.x = self.x;
			bullet.y = self.y - self.height / 2;
			game.addChild(bullet);
			heroBullets.push(bullet);
			self.shootTick = self.shootInterval;
		}
	};
	self.update = function () {
		if (self.shootTick > 0) {
			self.shootTick--;
		}
	};
});
// Define the HeroBullet class
var HeroBullet = Container.expand(function () {
	var self = Container.call(this);
	var bulletGraphics = self.createAsset('heroBullet', 'Hero Bullet', 0.5, 1);
	self.speed = -10;
	self.move = function () {
		self.y += self.speed;
	};
});
// Define the Zombie class
var Zombie = Container.expand(function () {
	var self = Container.call(this);
	var zombieGraphics = self.createAsset('zombie', 'Zombie enemy', 0.5, 0.5);
	self.speed = 2;
	self.move = function () {
		self.y += self.speed + Math.min(Math.floor(score / 10), 5);
	};
});
// Define the Background class
var Background = Container.expand(function () {
	var self = Container.call(this);
	var backgroundGraphics = self.createAsset('apocalypseBackground', 'Apocalyptic background', 0, 0);
	backgroundGraphics.width = 2048;
	backgroundGraphics.height = 2732;
});

/**** 
* Initialize Game
****/
// Create and add the background to the game
var game = new LK.Game({
	backgroundColor: 0x000000 // Init game with black background
});

/**** 
* Game Code
****/
// Create and add the background to the game
var background = game.addChild(new Background());
var hero = game.addChild(new Hero());
hero.x = 2048 / 2;
hero.y = 2732 - 100;
var heroBullets = [];
var zombies = [];
var score = 0;
var scoreTxt = new Text2(score.toString(), {
	size: 150,
	fill: "#ffffff"
});
scoreTxt.anchor.set(.5, 0);
LK.gui.top.addChild(scoreTxt);
// Handle touch and keyboard events for hero movement and shooting
game.on('down', function (obj) {
	var touchPos = obj.event.getLocalPosition(game);
	if (touchPos.x < 2048 / 2) {
		hero.moveLeft();
	} else {
		hero.moveRight();
	}
	hero.shoot();
});
game.on('keydown', function (obj) {
	var key = obj.event.key;
	if (key === 'a' || key === 'A') {
		hero.moveLeft();
	}
});
// Main game loop
LK.on('tick', function () {
	// Update hero
	hero.update();
	// Move hero bullets
	for (var i = heroBullets.length - 1; i >= 0; i--) {
		heroBullets[i].move();
		if (heroBullets[i].y < -50) {
			heroBullets[i].destroy();
			heroBullets.splice(i, 1);
		}
	}
	// Spawn zombies
	if (LK.ticks % 120 == 0) {
		var zombie = new Zombie();
		zombie.x = Math.random() * (2048 - zombie.width) + zombie.width / 2;
		zombie.y = -zombie.height / 2;
		zombies.push(zombie);
		game.addChild(zombie);
	}
	// Move zombies and check for collision with hero
	for (var j = zombies.length - 1; j >= 0; j--) {
		zombies[j].move();
		if (zombies[j].intersects(hero)) {
			// End the game if a zombie hits the hero
			LK.effects.flashScreen(0xff0000, 1000);
			LK.showGameOver(score);
			return;
		}
		if (zombies[j].y > 2732 + 50) {
			zombies[j].destroy();
			zombies.splice(j, 1);
		}
	}
	// Check for bullet-zombie collisions
	for (var k = heroBullets.length - 1; k >= 0; k--) {
		for (var l = zombies.length - 1; l >= 0; l--) {
			if (heroBullets[k].intersects(zombies[l])) {
				heroBullets[k].destroy();
				heroBullets.splice(k, 1);
				zombies[l].destroy();
				score++;
				scoreTxt.setText(score.toString());
				zombies.splice(l, 1);
				break;
			}
		}
	}
});

===================================================================
--- original.js
+++ change.js
@@ -44,9 +44,9 @@
 	var self = Container.call(this);
 	var zombieGraphics = self.createAsset('zombie', 'Zombie enemy', 0.5, 0.5);
 	self.speed = 2;
 	self.move = function () {
-		self.y += self.speed;
+		self.y += self.speed + Math.min(Math.floor(score / 10), 5);
 	};
 });
 // Define the Background class
 var Background = Container.expand(function () {