Not bad
HP shouldnt be the only determining factor to these games, resist, speed and maybe health regen if you want to get tricky with it, should be factors in determining the diffculty of the ai.
More towers, its been said too much.
BEEF, let me say that again BEEF, the boss waves. They were much much easier then the other waves which was rather disappointing.
Try and find a balance in the cost of the towers and the money earned, in round 5 I sold off all my towers and bought a couple missile towers, after that all i did was buy more (which I think I stopped with 14 or so) and upgrade them.
All in all it wasnt bad, just a bit bland.