From now until May 13, Steam is offering Defense Grid: The Awakening for 75% off. At $20 the game is a bit of a hard sell, but at $5 I say you should go for it.

Here's the official description from the Steam Store:

 Defense Grid: The Awakening is a unique spin on tower defense gameplay that will appeal to players of all skill levels. A horde of enemies is invading, and it's up to the player to stop them by strategically building fortification towers around their base. Beautiful environments, spectacular effects, and a dynamic, engaging soundtrack bring the world to life. The controls are intuitive and the gameplay is deep — the special attacks and properties of each tower work together to provide many ways to succeed.

• High Replayability There are approximately 8 hours of gameplay in the main storyline, and many hours of play in challenge modes that give players unique starting conditions and objectives. Each game level is very replayable, and can be solved in many different ways, with increasing rewards for improved efficiency.
• Wide Variety of Enemies Over the course of the game, players battle 15 different enemy types that become increasingly stronger, and employ a variety of strategies in an attempt to bypass the player's defenses. As the levels progress, the enemies become tougher and more difficult to defeat.
• 20 Unique Levels Defense Grid: The Awakening has 20 unique environments, each with a different placement of roads, tower build locations, and open areas to plan a strategy around.
• Numerous Tower Options and Upgrades There are 10 different tower types with 3 levels of capability each. Each tower has unique trade offs that affect ideal placement, such as line-of-sight attack or ballistic trajectory fire. Some towers improve other nearby towers, and some are ideal at specific locations, such as a rear guard tower that unleashes a devastating attack.
• Steam Achievements Successfully completing each level provides a bronze achievement award. Additional goals provide silver and gold awards for a particular level. A novice player may be able to survive the onslaught, but only an experienced player will be able to get all of the gold achievements.

$5 is a steal. Buy it. It's a great game.

Awesome. Great deal.

The cores. They're making off with them!

You said tower defense and I got excited. I saw 3D and closed the window.

I just bought this with no idea what this game is about. That's a great price so I'll take this recommendation. Thanks for the heads up.

Looks cool. I might check it out.

"Looks cool. I might check it out."

Well I got it, and I'm really enjoying it. Played it for about an hour, finishing the first 4 levels and got shit loads of achievements. It's really cool.

So I guess I have more people to compete with for high scores now. :)