It's tower defense Borderlands 2.
The first Sanctum, developed by Coffee Stain Studios, launched with an intriguing hybridization concept: take familiar first-person shooter elements and snipe them into the tower defense genre. Released on Steam in March 2011, this single-player and cooperative title received fairly positive reviews for its uniqueness, though it was on the short side, essentially a prototype waiting for deeper development. Coffee Stain Studios heeded the advice and the result is the upcoming Sanctum 2.
Similar to its predecessor, the object of Sanctum 2 is to prevent waves of monster from breaching your defenses; in this case, protecting the central core from taking damage. Each progressive wave increases in difficulty, with additional monsters and stronger ones at that, but you've got more than enough firepower to manage.
During the construction phase, you can build blocks in designated rectangular grid-based areas to slow down or outright destroy enemies before they even reach the core. The pathways of how monsters will appear during each wave is shown on your handy map, and by placing blocks in strategic places, they will need to go around the blocks, all while taking fire from turrets purchasable and upgradeable through resources collected between each wave. There's something dark, yet satisfying, about killing enemies by using the item drops of their fallen brothers.
The only downside is that however you build your defenses, you can't completely construct blocks in such a way that monsters have no way around them; or, in other words, a wall. But worse case scenario, if enemies do start climbing over your defenses, you can take care of them yourself. All it takes is a few well-placed shots with your gun of choice and the suckers don't stand a chance.
Better yet, in this sequel you can join with up to three extra friends in cooperative play on fifteen maps, hopefully as different classes to take advantage of each other's strengths since the monsters becomes more difficult with more players. Most players will likely choose the straightforward Soldier class before spreading out to the Tank, Demolition expert, and Sniper (my preferred class), all of which gain experience and can unlock various perks. One in particular that the producer of the game showed me is a damage shield around the core, so that pesky grunts will commit suicide if they manage to get through. Beyond that, this makes it easier to assign the best role to each player.
Sanctum 2 is set to release in May 2013 for not just Steam, but also Xbox Live Arcade and PlayStation Network, for the low price of $14.99.