Amesome
I've played a few defence games on this site before, but this is definately the best one. The RPG elements help you decide how to upgrade your 'towers', while still giving the opportunity to build a wide-spread array. The massive monster list is always interesting as well. (I especially liked the "Spector Corpor(i)als")
I have two problems, however. First: the Paladins and Necromancers arn't available here. It drives me nuts when the designers make us go to another site to play the full game. Second: four of the five basic units are the same. If all you could come up with was a bunch of mages, you could at least come up with different names for them, and not just colors.