Nice job
I can only think of a one big game ruiner. Made it to level 12, but didn't kill everything before the buy screen came up. The result was an enemy pounding on the castle that would not die, even while in the buy screen. Fix that glitch (probably occasional) and I can't think of anything else I'd do to improve it.
Nice work!
And for the people who have trouble with level 5: Rate of fire. Can't have enough of it. That and 2 levels of damage, and those giants fall like trees.