Super Sanctum TD is a tower defense game by Coffee Stain Studios similar to the main Sanctum franchise. The main difference between the two is that Super Sanctum TD is a top-down flash styled game with graphics from the 16-Bit era whereas the main series are both first-person shooters.
The plot is still generally the same; you protect a core on various levels from waves of aliens known as Lumes. The Lumes, in turn, want to destroy the core. The game has two modes of play, Standard and Survival, in two difficulties, Normal and Hard. Standard mode lasts usually 10 waves of Lumes to defend against, and Survival is a never ending onslaught of the beasts until the core is destroyed. Hard mode gives the Lumes more health and they deal more damage to the core per Lume.
You are given various towers from the Sanctum franchise to defend yourself from the onslaught, ranging from Gatlings to Violators, Slow and Amp Fields, Kairos and Lightnings. Each tower gives a different effect to the Lumes, and differs in damage depending. Finding a good strategy between them all is key to your survival. Towers can be upgraded to level 3 as usual, but cannot be overcharged. Killing Lumes gives you money and experience depending on which Lume is killed.
Instead of weaponry to defend a player character, you play as an omniscient tactician, given an aerial view of the whole map. This also means you don't get any weapons from the franchise. Sadly, no Rex. Instead, you get various skills that can "attack" the Lumes such as Mines, Pulse Fields, and Black Holes to hinder their progress.
You can upgrade your towers in a different way by upgrading Ability Tiers when you level up. Each level gives you one Ability Point. Spending these allow you to power up your towers, such as having Physical towers deal random crits, having slowing towers deal damage per second based on slowing, etc.