Fun game, pretty hard to be honest! The music can get a bit repetitive, but it wasn't too bothersome - at least it seems to fit with the game's tempo. Too bad there aren't more levels, but I really like the concept. Perhaps if you were to redo this you might want to
a) add more/different music, perhaps different music for each level?
b) more levels
c) the tank gun should probably rotate in the direction of the mouse, would definitely look better