If you're a fan of tower defense games then Defense Grid: The Awakening will appeal to you with in ways that similar games may not.
The story is thus - a horde of enemies is invading, and it's up to you to stop them by strategically building fortification towers around the base. Your defense consists of a series of small towers that you can erect in the area prone to attack. These towers can be setup to destroy anything that comes their way with firepower such as machine guns, long-distance lasers, flame throwers, missiles, explosive projectiles, electric screens - you name it, Defense Grid: The Awakening has got it.
The good thing about this is that it doesn't require much tactical or strategic knowledge and you can get going right away. The bad thing is that it can get rather boring - when all you can do is blow away enemies, you soon get very frustrated when one makes it through your line of defense and there's not much you can do about it other than reassemble your towers.
The variation and challenge however lies in the number of different scenarios that are available. The lie of the land has a large bearing on the difficulty of the defending your towers. There are 20 levels to choose progress through to keep it interesting although it's still a very limited strategic game.
If you're not interested in complex tactics and stunning graphics, Defense Grid: The Awakening will provide you with a few hours of defensive, alien blasting fun.