I'll be sincere: I was a bit skeptical about this game. However, turns about this is a pretty good game!
It is creative and well implemented, being very simple yet interesting. The powerups sure help, changing the scenario from "almost losing" to "the screen is clear again".
However, I think there are two flaws, both quite minor (just my opinion, of course). First, I think you could clamp the cannons speed, and second, the gameplay gets a bit repetitive after some time. Either way, pretty solid for a 72hrs jam!