There's something uniquely addictive about tower defense games, which is the only real problem I have with Defense Grid. I meant to get things done this week, but instead, I've been wiping out entire armies of aliens.
The plot's appropriately paper-thin. A thousand years ago or so, an alien race invaded the planet and were driven off in a cataclysmic war. Now they're back, and your only hope of stopping them is in reactivating the ancient defense grid that protected the planet last time. To help, you have a slightly befuddled veteran of the war, who uploaded himself into the system in case the aliens returned.
Defense Grid is mostly about trial and error, which is both frustrating and a big reason why it's so addictive. Aliens enter the map from a set point, and you construct towers to either make their path through the map as difficult as possible, or to actually create a lengthy gauntlet for them to traverse. The aliens' chief advantage is sheer weight of numbers, so that no matter how many towers you put in their way, sooner or later, a couple of them are going to get through.
It's pretty simple to learn how to play the game, and it's satisfying to construct an increasingly complex and powerful series of towers. Your nameless AI buddy adds to that with one-liners and constant narration, telling you whenever your defenses have managed to take down a boss alien. There's a definite learning curve at work here, and your second path through the game feels ridiculously easy compared to your first. You can also hit up a number of optional challenges, such as more powerful enemies, flipped maps, or endurance rounds.
There are a couple of minor irritations that keep cropping up, though. For one thing, Defense Grid doesn't give you much in the way of panic buttons; you have exactly one, an orbital laser with a ridiculously long cooldown. When your AI helper is shouting at you to target a specific boss alien, and you actually have no way to go specifically after that one, it's a bit annoying. It's also ridiculous that concussion towers do almost no damage until they've been upgraded twice.
The trial-and-error gameplay could probably be mentioned here as well, but that's really part of the genre as much as anything else. My real problem with Defense Grid only starts cropping up near the end of the game, when everything and its mother starts showing up with shields.
Shields are an annoying alien gimmick which prevents the alien from taking damage from heat-based attacks at all, and which essentially doubles the alien's health against any other forms of attack. Most of the area-of-effect towers in the game are heat-based, such as the inferno. In the latter half of the game, it's not uncommon to see entire squadrons of aliens being supported by a Spire, which shields everything around it.
You can sometimes counteract this with a fully-upgraded meteor tower, to be fair, or with a temporal/level 3 concussion combo. When you first encounter them, though, it's hard to shake the notion that the game is cheating somehow. It's set you up with a blatant disadvantage - the only towers that can consistently damage shielded aliens are all single-target - and is now exploiting that for all it's worth.
Maybe that goes back to the game's odd learning curve, though. Defense Grid takes about five minutes to learn, then slowly gives you more tools over the course of campaign mode. Even with that, though, the game expects you to arrive at various tactical conclusions on your own, then exploit them to their utmost extent. It's not quite a fair game, but it's one that kept me coming back again and again for a week. Maybe that says more about me than it.