In some ways, it's fairly typical: you set up weaponized towers to kill the incoming enemy to prevent them from reaching some kind of goal. In this case, the enemy's goal is to steal your fusion power cores and get them off the map. Once gone (or once picked up by flyers), the cores are irretrievable and in most cases your chances at higher medals are gone, too.
There's a simple storyline as well, but it is helped along by an AI named Fletcher, voiced by Jim Ward, a prolific voice actor. He provides the plotline, explanations of the enemy, prompts when certain enemy types appear or when enemies grab a power core, handy when you're dealing with something in one corner of the map and can't see what's happening on the other side. The maps are 3D with zoom (though no rotation), but while it's smooth and the graphics are good, I rarely use the zoom at all.
As with most tower defense games, you're somewhat limited where you can put your towers, though where a tower can be placed, any available tower may go there. Placement is often fairly open-ended, allowing you to create paths that the aliens must walk, and this becomes critical in many places where you have to deal damage over a longer period of time. Block the path to the cores completely, though, and they'll walk right through the towers (this is expected and you get warned about it).
There are ten tower types in all, each with two upgrades levels:
- Gun: A simple autocannon, usable against ground or air targets
- Inferno: A short-range flamethrower
- Laser: Just what it says
- Temporal: Sends out a blast that does no damage but slows the enemy
- Meteor: Shoots a ball of flame in a ballistic arc over a long range
- Missile: Surface-to-air missile site; doesn't target ground units
- Cannon: Heavy artillery; slow to fire but hits hard
- Tesla: Discharges an arc of electricity; more powerful if it can charge for a bit
- Concussion: Rotary grenade launcher, useful where the enemy is going around turns
- Command: Sees hidden units (which you can see but towers can't until they're close), recovers extra resources
Combined with all the maps and the challenge modes for the maps, there's a lot of game play. It's not uncommon for me to spend an hour or so on a level, though some have consumed much more. Scores are uploaded to a central server, though it's clear that some people have figured out how to upload fake scores.
There is some DLC available that adds new maps and new voice recordings, and one additional DLC called You Monster that features Portal's GLaDOS in recordings made specifically for Defense Grid. In it, she does pretty much what she does in Portal/Portal 2. (If you've never played even Portal, go play it now.) It makes for an interesting twist and does make you want to beat the level even more.
The game is $9.99 on Steam. The four map packs are $.99 each and the You Monster DLC is $4.99. A complete package with everything is available for $15.99. For those that don't want to pay this much, it's a frequent sale item with Steam, so add it to your Wish List and you'll get a notification when it's out.
In any case, if you're even a passing fan of tower defense, I wholeheartedly recommend it.