It's an interesting free-form (ie, no true paths) tower-defense game. Pretty large tech tree, pretty long game, pretty hard, and it under-went a major gameplay-overhaul not too long ago. I'm not convinced it's better, but it did noticeably simplify a few things which were over-complicated. And large changes / updates are generally indicative of developers who listen. As is the overall level of polish, which I'd call abnormally high.

Definitely worth a try, unless you dislike tower defense games.