It's not often that when I get tweeted a game I look at it and think "instant buy!" well Build & Defend is one such game.
You wake up in a desolated and dangerous world, where almost everything is a deadly trap. Will you be fast and clever enough to survive? This game mixes several classical gameplay-styles such as Tower Defense, Multi-player cooperation, Destructible world and Economics
I have tried out the tutorial of the game and I think it's awesome, the controls are a little odd and take getting used to but after that it's actually really fun to play.
You can play for free with limited options unless you fork out and pay the $4.90 for the full game, seems like it's really worth it too.
Something cool about the game is it's in-built "social panel" where you are always logged into the main chat room for the game, could have a lot more uses though.
I would give this one a go, it could become huge if given enough support.