Defense Grid is a science fiction tower defense game with awesome graphics and a nice variety of towers and alien enemies. It's great because it involves strategy: there are only a few places you can build towers, so you have to exploit the best locations and balance building new towers with upgrading old ones.
The excitement of watching aliens run off with my power cores nearly had me yelling at my towers to just shoot down one more alien to get enough resources to buy a tower upgrade for that extra bit of alien-annihilating power. A handy feature is the fast forward button to detract from the boredom of waiting for your towers to finish off the aliens.
Overall, the game is addicting and enjoyable because each level features a unique location, more and more challenging aliens to fight off, and more powerful towers.
-Rebecca