Like the bit of parallax you had with the BG. Some things you could have done to make gameplay smoother is instead of checking on a timer if the shoot button is down to shoot, when they shoot, start a timer, and then don't let them shoot again until it's up. That way when you wait for a while, then shoot, the bullet comes out instantly. That better suits the idea of paying for shots as well.. About that, it would be nice if there's immediate visible feedback of your resource count going down as soon as you fire a shot. As it is, you see it going down, but it doesn't really feel like it's linked to you firing.
My initial thought was that you should add a delay to prevent them from selecting a level up ability as soon as the screen came up, so the player doesn't accidentally select a powerup just because they're holding in the fire button. Well, that's not great if you level up a dozen or hundreds of times at once.
An end condition trigger in that case would have been nice. Turns out multiply drop rate is pretty good:
