Probably one of the most unusual tower defense games I've played, Creeper World took me completely by surprise becomming one of my favourite games of 2009.
Rather than defending against various small to large attackers you defend against a single entity, the creep. There was something spooky in the way it would slowly but surely make it's way over to you, little by little, calmly flowing over obstacles until it engulfed your base, the final refuge of humanity, Odin City.
Of course you did have towers which could defend against it, but you always get the sense you're fighting a loosing battle and every level, aside from the last, involves you powering up a warp gate to escape!
The good news for people who've not yet played it is that there is now a completely free version of the game called the training simulator. You can play that here, which you must now do otherwise all is lost!
However over the last couple of months something new has been emerging from the brain of Virgin Wall, creator of Creeper World. And that is, as you may have guessed: Creeper World 2! Check out this awesome video of the editor and also, later on, the new game in action.