Freezes
It would have been a great game, but half-way through a wave, after I upgraded something, I couldn't fire anymore (see "fucian"'s comment a few below mine). The game was receiving keystrokes, but not detecting the mouse at all. (I didn't take away stars for that.)
Other than that, it was a nice game. It was a bit slow as defense games go, but that allows the player time to decide a strategy against the oncoming waves, particularly since the number of possible weapon-combinations kept increasing. It kept my interest. I thought the upgrades were a little wonky, though: I would click a Lvl2 upgrade for one item and the price of ALL Lvl2 upgrades for anything else would also increase exponentially. Odd. Keep working on it, I think, but it's a good start.