Very complete game, lots of features and very well polished. What grab my attention was your interpretation of the theme, very different then most submissions, extra credit for that.
The only thing I would like to be improved is that sometimes cannons shoot even outside of the screen. I getting that when its just coming in/out of the screen it could fire, but quite often I had to dodge a projectile coming from a place outside the screen, specially worse when it comes from the left because you naturally assume that there would be no thing coming back. That part looked a bit unfair to me, but it is a very small detail on a great game overall.