Sanctum (2011 video game)

An Indie Game released on Steam by Coffee Stain Studios. It's touted as the first Tower Defense-First-Person Shooter, which adds a dash of frantic-ness to the usual Tower Defense fun, as well as a Co-op mode, where you can defend your tower (or in this case a Core) with buddies!

Enemies come in many flavors, most of which are Exactly What It Says on the Tin:
 * Walkers - Jack of All Stats
 * Tanks - Mighty Glaciers
 * Runners - The Not-So-Fragile Speedster
 * Floaters - The Space Stingray

And more! There are also at least four types of defenses you can build on top of your blocks, as well as three weapons that all need to be upgraded to win the continual Rat Race against the continual waves of enemies.

Sanctum contains examples of:

 * Action Girl: Skye, the main character
 * Attack Its Weak Point: Most of the creeps have parts which are either the only vulnerable spot to them or hitting there does more damage than hitting elsewhere. Weak points are marked by glowing.
 * Back-to-Back Badasses is what you and your optional buddy(ies) can become when you're surviving that twentieth wave against Big Walkers.
 * Boom! Headshot!: The sniper rifle
 * Bottomless Pit: Near the edges of several stages.
 * Crossover: The Yogscave free DLC level.
 * Dance Party Ending: Skye likes to shuffle or do the Caramelldansen after her victories. The same happens if the core is destroyed, only with Walkers dancing instead of Skye.
 * Excuse Plot: Skye is sent to Elysion One to stop invading aliens.
 * Frickin' Laser Beams: Laser towers.
 * Macross Missile Massacre: One firing mode of the newly released REX Launcher
 * More Dakka: Gatling Towers
 * Scenery Porn
 * Shock and Awe: Lightning Towers
 * Shout-Out:
 * In the achievements, there are several. There's a whole collection of them relating to The Protomen, as well as one called Giant Enemy Crab.
 * There are references to I Love Strawberries, a failed game that the studio previously worked on.
 * There Is No Kill Like Overkill: The achievement "I Said STOP!" requires the player to shoot an enemy in a slow field with a freeze grenade.