Unless you've watched the recent developer interview with Hidden Path on Xbox Live, you may not yet be aware of this. In that interview, the developers at Hidden Path talked openly, albeit briefly, about being deep into a XBLA suped-up version of Tower Defense.
I about jumped out of my seat with joy upon hearing this. Master of Defense was one of my favorite games of 2006 and it was essentially a prettied-up version of Tower Defense available on the cheap for the PC. I think it cost $10 but I got tons of enjoyment out of it and wrote at lenth about it on this site... gentlemen start your search engines.
Having never played the original TD, I went looking for it online last night to make sure it was one and the same with my much-loved Master of Defense. It is. The games are very simple in design: you position various towers on the screen to shoot and destroy the streams of enemies moving from left to right or top to bottom. There are various types of enemies and various types of towers that can be positioned on the screen, then upgraded, then upgraded some more. It's extremely simple and impossible to put down.
I probably shouldn't admit this in case my editor is reading, but I got home from last night's road ride (20 miles, 2800 feet!) totally exhausted and wanting to go to bed. The plan was to play one or two quick games of Tower Defense then either get some work done or hit the hay. I ended up playing, trancelike, for over two hours.
And you can join me.
Here is an excellent flash-based version of Tower Defense that you can play for free that is not only very cute (hand-drawn graphics and mouth-spoken sound effects) but plays well. It's free and also has a feature to upload scores to a group scoreboard. Not only that, but you can browse other people's scores and actually see maps of their tower placements to get an idea on how to score more points.
So, while the XBLA version of TD is probably at least several months away, we can enjoy a little friendly competition with this version right now.
To enter scores onto the group leaderboard, you must play on at least Medium (normal) difficulty. Finish your game, enter your name into the box and hit send. A screen will open asking you to upload to a group. Once you do, type "Randomly Generated" into the box and your score will be listed among my own and, potentially, other ambitious RG readers.
Enjoy! Just don't be mad when your productivity plummets.