This game could have been really really cool, and it sort of disappoints me that the cannon wasn't easier to control. I love the charm and humor of the thing.
I would make it so you don't need to push the cannon to move it. If you could just move the cannon directly it would be way more satisfying to play.
Also give the player some time to get adjusted to the controls early. Enemies start flying at you from all directions right away, and I'm just barely catching my breath while figuring out how to control my guy.