Good start
I think of this as a good base to build up from to make it a better game.
Problems:
-The cannon didn't follow the mouse so you had to wait until it got there before shooting, otherwise you missed.
-The enemies were sometimes hard to click on and you had to click 3 or 4 times occasionally for the cannon to fire at it.
-The system for upgrading was flawed; it required 25 kills but this meant you may aswell kill 25 small pathetic enemies and get an upgrade than kill 5 more powerful enemies --- If you changed the upgrade system to killing things for money and purchasing upgrades it would be much improved.