Sanctum, a 2011 main game for PC and Mac, uniquely fuses tower defense strategy with first-person shooter action. You play as soldier Skye, building and upgrading automated defenses to stop alien waves, then jumping into the fray to fight directly alongside your towers. Its core appeal is this mandatory integration: success requires both tactical foresight in tower placement and precision in combat. The game features one DLC expansion.
The dual nature of the gameplay means success depends equally on foresight in tower placement and precision in first-person shooting. Players must constantly adapt their defenses based on the types of enemies encountered and the flow of the battle, often jumping directly into the fray to support their automated defenses where they are weakest.
The defining feature is the mandatory integration of the first-person shooter perspective directly into the tower defense framework. Unlike traditional tower defense games where the player is an omniscient observer, here the player is an active, armed participant on the battlefield, directly influencing the outcome through combat skill alongside strategic building.
The primary objective is to ensure the survival of the core energy source on each map by building an effective network of defensive towers and actively participating in combat to eliminate all waves of hostile alien life forms before they reach their target.
Regarding additional content, the game features one downloadable content (DLC) package available for purchase, which expands upon the core experience with new challenges or additions, though no full expansions have been released.

This map pack adds three action packed levels to your game!ChasmThe Chasm was going to be a hub of transport and mining operations. The plan was to create thousands of new jobs and strengthen the living standards of the people of Elysion One. The Chasm was a good sign of a great future for humanity but nowadays it's abandoned.

This DLC adds four additional maps for Sanctum. The maps included are: Aftermath, Aftershock, Cavern and Slums.

After the initial onslaught, a decision was made to take the subterranean rescue efforts, to somewhere free of the alien infestation. Scientists had recently discovered that the strange lifeforms that had been assaulting Elysion One would only venture so far into the earth. While this is good and all, there remains one small problem: someone has to keep them at bay while the operation is moved deeper into the earth. Guess who will be part of the lucky battalion? This DLC adds the 'Caverns' map to your library of accessible content. With this DLC, you will be able to host games on this map and engage in single-player conflict.

Coffee Stain Studios have teamed up with The Yogscast to bring their signature 'Joffo Cake' fuelled humour to Sanctum in the free DLC, Yogscave! As the ruling elite in Elysion One, the brightsiders live a life of decadence but this luxury comes at a terrible cost to the less fortunate members of society, who toil in factories below the surface - securing minerals to craft weapons to defend against invaders. Simon and Lewis are two miners who have found themselves on the night-shift to end all night-shifts. After Simon's "amazing" idea to increase Joffo Cake storage by 46% by diverting power from the core's energy containment shielding, the night was off to a great start! Unfortunately this idea backfired as the reduced shielding attracted unwanted attention from some of the Yogscave's more sinister denizens. The invaders are different this time, they appear to be organised by some far more nefarious mind. Experienced at dealing with the odd Barret Bat and Blessed Boar here and there, Simon and Lewis are still unable stop these invaders and require your help! Can you save the world? And more importantly can you save the Joffo Cakes?! This DLC adds the 'Yogscave' map to your library of accessible content. With this DLC, you will be able to host games on this map and engage in both single and multi-player conflict.