Very nice game, looks really good and feels smooth when playing! I'm guessing in the solo mode you don't get to use the cannon, but if it's the same for multiplayer (4 guys shooting to the center) what would worry me is the balance, given the cannon's fire rate (feedback based on the solo mode, in case it changes!). For the solo mode it's fine, but humans usually come up with better, different strategies when shooting than just aiming at the player like the AI does.
One thing, I don't know what's up with that, but i think that it was most likely a unintended behaviour: I've shot continously one of the cannons and it started spinning like a beyblade, then it didn't know where to aim xD
Great job with this one, I'll keep it around and see if i can find people for a multiplayer session, cheers! :)