This is really well designed and thought out, especially considering the time constraints. The art and the presentation are really clean too.
In terms of feedback, maybe consider allowing the cannon to fire even when its against a wall. I think that could open up more puzzle opportunities for your game. (Speaking of which, if your game does get featured, I wonder if Mark will make a joke about shooting to move considering he once joked that a future theme should be "Shoot to Move." Not that it's a bad thing.)
Anyway, great job. :)