Yesterday I logged onto Steam with the intention to try to play Left4Dead today. In doing so I noticed you could look at the history of people on your friends list. I looked at one of my friends history and noticed he had been playing Defense Grid: The Awakening. He had in fact been playing it quite a bit (could see lots of achievements). Since it seemed to be a tower defense game I bought and downloaded it to give it a try.
12 hours (and an episode of BSG) later I was still playing! (basically all of Sunday)
The game starts of simple, introducing a few basic towers and keeps your options limited so that it is fairly easy to win. As the game progresses you gain access to more interesting towers and more upgrades and options. The game then gets much harder, with many different aliens to kill and more options to create paths for them (which you will need in order to have enough time to kill some of the harder enemies)
All the way through the game you have a guide, some sort of AI who was transferred from a human brain into a computer. He gives advice early on, and starts to get a bit emotional as the game progresses. The voice over certainly adds a portal-esque feel to the game which makes it all the more enjoyable.
This game is definitely worth the money (20 euros) and while I have almost reached the end of the story, there are also challenges for each level and a leaderboard so you can see how well your friends did and try to beat their score.