It's okay.
I'll cut you some slack, as this is obviously one of your first TD games, but this is kinda crappy, considering how much it lags even with low settings, and how weird the tower upgrading is. The "stealth" enemies really don't do anything at all, as your towers will still shoot them when you can't see them. The ghost enemy types seem to need to be right next to your normal towers in order to hurt them, and the game becomes a jumbled mess after a while. Upgrading towers is impossible as soon as you hit play because the enemies come too fast (even with the lag :P). It's okay, but definitely not something I'll rave to my friends about, or come back and play a second time.