Defense Grid takes the tower defense genre to the next level. I liked every aspect of the game. Level design, tower design, tower diversity, enemy types, economic system, soundtrack, effects, difficulty level, extra playable modes, the fact that tower placement shapes the aliens path. Everything is designed very carefully and delivers a perfect playing experience.
I got all achievements and I'm still playing to earn the gold metal on every single mode of every level.
I admit that the first time I played DG I looked down on it because it looked like a "small" game. That was on a friends computer before I finished the main campaign. Then I played it again a year later on my brothers computer and finished the main campaign again! Finally, this summer, I got it for myself and finished it again, plus every expansion that was available.
I'd like also to note that the voice of Jim Ward, who voices the AI assisant, is a big plus for the game. It gives you the feeling you're together into the fight, with a person that shares his personal moments with you, even his craving for raspberries!!
The Containment, You monster, Resurgence and Borderlands expansions add greatly to the game, in playable levels but also in the story.
And above all else is the Strategy. You may beat easily a level the first time, but it's certain you will come up with a better plan the second time you play the same level. And different game modes (like "out of bullets" or "shredder") will force you to adopt a completely different approach to beat the level. And all of this is done by placing just 10 different types of towers on those tiles. The game depth is incredible.
I give this game a 100% with honors! An excellent idea, a perfect design into a superb game.