It's free, highly addicting... and overall a great game. If you like tower defense, you'll love this game.
I got up to 31,000 score on that game! I just started to "replace" items with better upgrades when I lost! My previous game I only got to 7K, so it definitely can be addicting. Those tower defense games are great. I have a few ideas for one myself, but I got some learning to do before I can make it so.
just search the market for "defense", there are SO many! Robo Defense is definitely fun, especially since its now 2.0 compatible. I've been meaning to try some of the others also.