Very fun game. I'd love to see more of it. Sounds and music were a little bit too loud, but managable.
As for the changes i'd reccomend changing the look and placement of the cannon cool down time, for example as a slider above the cannon. It was pretty hard for me to notice that i was charging cannon while holding LMB, and it was even more difficult to see the cool down.
Overall, solid game. Needs a little bit of polishing but solid and fun to play.