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.

It is awesome... They did make a lot of changes (like 12 or so new versions since then) so you should fetch the latest when you are ready to give it a try. The whole gameplay is different, basically they released a "pretty good" game for the bundle #2, then iterated with feedback to get to "awesome". Got to admire this attitude.

Also played their Droid Assault, sell it on their site in a bundle as well. It's an instant hit, could hardly stop playing, just love it :) http://www.puppygames.net/droid-assault/