Very nice. Even has an online map editor.
I get the feeling from playing it that there is a lot of strategy that could come into play. Coders clearly had fun with this one and implemented some very cool stuff.
I am also very impressed with the way they did the "Hero" part of it. Basically in addition to the normal stationary tower defenses, you have a "hero" you can move around on the screen. Normally i really dislike this idea since i dont like to micro manage in a TD game. But this is a great compromise because you dont target or shoot for your hero, which means you can basically just move your hero someplace and leave them there for most of the time. So very minimal guidance is needed.