Plays much like a certain other game.
Well, I don't have anything to say that hasn't been said in reviews before me. The game is obviously inspired by "Defend your Castle," and has the same strengths and faults. It's fun and easy to play, and the upgrades are neat. But it also becomes boring and pointless once you get the best defense options. Computer lag ensures the action never becomes too fast, and so you're never overrun with too many enemies.
There's really not much you can do about the gameplay mechanics; if you make the defenses weaker or the enemies harder, it will either stay boring or get too hard. You could have more enemy types, but that only delays the inevitable.
Some ideas:
Sound on/off button. The effects quickly become annoying.
stronger bullets/guns for the player.
Bigger screen. This window size is kind of small, but I suppose that improves game performance.