Your game is awesome! I loved the art style, the music and sound fx and the gameplay. Really good game, I've had a lot of fun with it.
One thing that should be improved is balancing the tank classes. I survived the 6 waves only buying a bunch of fighter tanks. The sniper tanks takes very short distance to fire, and its damage isn't so high. The ultra tank dies very quickly and is very expensive. The bombs are very useful, maybe make it a little more accessible and reduce its range or damage.