What this game is lacking is upgrades for your fort and gun. Having the Rampartesque cannonballs is fin for the first few levels, but it's too big a limitation. Add some upgrades like faster firing time, more shots in the air at once, maybe some archers in the battlements that fire weak shots automatically at the nearest enemy.
Thanks, you are completely right, I'll do that in the sequel.
Curious mate, I like it