Update: Sale over! I've updated the prices.

The tower defense genre may be crowded, but it's far from tapped out. We had a great time with Anomaly, which took tower defense and made you the invading force, and now Sanctum on the PC has become the latest game to get us defending our base from waves of bad guys. The twist here is that the game is played from a first-person perspective and, with the latest version of the game, you can play a co-op with up to three of your friends.

Playing in the first-person not only makes the action more immediate, but it also means you need to jump in and use your own weapons to fight off the hordes of aliens. The graphics are attractive, even if they're not stunning. The controls are simple and get the job done. This is a great way to spend the evening.

Control the flow of enemies, and then kill them

The trick is the ability to create a path through the level that funnels the enemies to your biggest and most effective guns. You do this while managing your funds to upgrade both your defenses and the guns you use yourself. Do you want to beef up your anti-air guns, or upgrade your own sniper rifle? You have to make hard decisions as you play, and an effective strategy often makes you feel like you're walking a fine line between control and ruin.

The enemies are another example of what makes this game so much fun. You'll be attacked from the air and from the ground; some are fast and some are very powerful. You get time to setup or upgrade your weapons and defenses before the enemies attack, and you need to take an active role in killing the waves aliens as they come in. You'll learn how the game works in the early levels when the enemies only come from one direction, but soon you'll have to juggle multiple points of entry. Even if you understand the basic concept of tower defense, it's not easy.

You have access to three weapons outside of your various turrets: a machine gun with a grenade launcher, a sniper rifle, and a gun that slows down or freezes the bad guys. Each weapon can be upgraded multiple times in the course of a stage, and you'll want to keep your own guns as powerful as possible; you can often save a bad situation by jumping into the fray and using your weapons effectively. This is even truer in multiplayer, and in games with two other players, we enjoyed running around and putting our bullets where they needed to go.

I don't feel the need to run down all the different kinds of turrets and their uses against the different kinds of bad guys—you can figure that out—but this is a fun game that has eaten up a startling number of hours in the past few days. Couple this with the fact that the game is only $15, and you have a purchase that should be a no-brainer.