It's a nice idea for a mobile game, I didn't really understand why the colors would change sometimes, gameplay-wise there was nothing new happening. At first I thought i was gonna be out of ammo eventually, but then the counter just continued to go up. I kinda wished there was more variety in it... little power ups, special powers for your crossbow like ice arrows that slow spiders down, or three shots at once, or fire arrow that damages after impact. Things like that could keep the fun going.
All in all it looks and feels good though, there is a lot of 'juice' ;)