Since a quarter of my hits seem to come from google searches about tower defense on XBLA, and I feel bad that the last news I gave is hopelessly out of date, let's diverge from our regularly scheduled MMO-discussions for a moment to discuss tower defense on the 360.
Right now there are three Tower Defense games on XBLA. I'm not really familiar with any "Live Indie" tower defense titles although they undoubtedly exist.
South Park Tower Defense
I haven't played the game the whole way through, this is just based on what I've seen in the demo. As far as I know this game is exclusive to XBLA, unlike the other two games.
Tower defense with a playable builder and set mob paths (no maze-building). You play as one of the South Park characters and can build and upgrade towers, while also collecting coins and throwing snowballs at enemies. There's a lot going on and you could almost call it an Action Tower Defense Game. There's multiplayer for up to 4 players both locally and over xbox live, though I didn't try out the multiplayer on live. When you have less than four players you can switch between characters and any non-player-controlled characters become mini turrets.
It's not easy to restart a level but the levels seem pretty short so far, so I'm hoping for no play-for-ten-minutes-then-lose-and-have-to-restart hijinks. I'm also not sure what tower variety is like, having only played the game a little bit.
I haven't played much of the game, but it appears solid. The playable characters are very welcome because I like to be doing something while I'm playing a tower defense game. I firmly believe that tower defense should never have nor need "fast forward" buttons. There's also no "interest" at the end of each round, something I hate as I think it's dumb for tower defense games to reward you for not building towers.
Achievements seem to be good, with no terrible Castle-Crasheresqe-Button-Smashing nightmares to be seen.
There's no new animation from what little I saw, just new voice acting along with stills. The game is wacky like the show but I'm not sure how much you would understand without seeing the show. There was nothing that really made me laugh from what I saw, but I'm hoping it's like the show -- a lot of exposition with a light smattering of fantastic gags.
Oh, and like the show it's not very kid-friendly.
Crystal Chronicles
A Final Fantasy-themed Tower Defense game made by Square Enix, which uses characters from one of the earlier Final Fantasy games, or so I've heard. Not being a huge FF fan I don't know if that's true or which game that would be.
There are three modes each with different "towers" -- towers being different characters that can be upgraded. Some modes also include upgradeable "crystals" which provide bonuses to characters close to them, which is kind of neat and pretty much the only unique thing about this game besides the FF theme.
Of course there is interest at the end of the level, which rewards you for not building towers and barely killing guys, and I hate that. There's also a fast forward button that you'll need to hold down from time to time (sigh) and to top off the evil features there's no partial rewind, so you'll need to restart every time you lose which, if you're me, is a lot. There seems to be a lot of strategic depth but you're often not sure why something worked or didn't work.
The game was released on a ton of platforms including mobile phones and it seems a lot like a game that was released for mobile phones.
Achievements are not fantastic, but they aren't terrible either. There is a high scores list but you'll need to manually "upload" your scores.
Defense Grid : The Awakening
You can probably get this game on Steam (for PC) for cheaper, but it is a great tower defense game and one of the giants of the small genre. If you like tower defense games and like the demo for this game I'd really recommend it.
It still has two of the bogeymen of the genre that I've dissed above -- interest and fast-forward. But it also has a checkpoint system without equal, as you can not only rewind to your last checkpoint, but back two or three or four checkpoints (though if you rewind past a checkpoint it's gone forever).
Tower variety is fantastic, and the upgrade system, though linear, is still very cool. The game looks great. There are many different kinds of enemies and a lot of strategic depth to the game, and unlike Crystal Chronicles it's usually easier to understand why you failed or succeeded.
This is also one of the few tower defense games I've played that has a variety of map playstyles. There are fixed path and maze making maps, as well as maps with different levels (heights) and some that are a mix of fixed path and maze making. The heights usually confused me, though, as there seemed to be line of sight issues, but at least it made the maps look cool.
There are a ton of game modes to try, and every mode/map pairing has its own high score list, which is awesome because you will occasionally get a high ranking if you did well on a map.