Comet Mayhem
However, the problems are this: One player was relatively easy to complete, even the additional DLC levels. On Versus (a part of the game I was immediately interested in) you will instantly know where to put towers, which ones to upgrade fully (pyramids for the ground, lasers for the airborn attacks) and the game ends up being a stalemate. My girlfriend and I were so proficient at Comet Crash that we ended up with a fully upgraded defence and pretty much a fully upgraded attack. I think the battle could've gone on for days but it just ended up being a farce. I actually think visually its a tie bewteen this and Savage Moon (tower defence wise) but the best one out there is still Pixel Junk Monsters (even if the title and graphics are a bit silly this game is an excellent challenge)
Worth a purchase but shame it misfires on a couple of fronts.