I agree, this is awesome. I've found that heavily speccing into Undead first makes for easy pickings. I mean, 40% fear on a level 4 tower? That's pretty sweet, right there. Then you get freeze, and the three-shot salvo at the beginning of a wave. Orc looks like the better one to go into next, then Demon. Although I have heard that investing in fully upgraded Meteor is "tha shiz." But anyway, I really enjoy this game, and not even the inability to reset talents matters much, it just adds a bit of thinking to the game.
EDIT: I noticed earlier today that this has become the highest-rated game on Kongregate, despite it only having been there for about a week. I mean, it's not hard to get a good rating on Kong, but this game definitely deserves it.