Everything breaks at wave 3-4. There's just a nearly unstoppable wall of enemies, and it spawns multiple waves at once. It would also be nice to have some visual difference between the 2 turrets and an indication of how much they cost.  The selling turrets also gets rid of ~3 instead of just the one you clicked on. Also, after you win you get a bunch of money and the waves keep increasing, even though the enemies don't move. Also, 2 small turrets are better than 1 big one, so the best strategy is to just spam small turrets.  The only way to "win" is to play the downloadable version, which gives you room for more turrets. If you place enough turrets before wave 4, you can stop the infinite clones and there's no difficulty from there.