The tower defence game in which you play an active role (which no-one seems to have come up with a snappy acronym for yet) is an increasingly popular genre, with upcoming titles like Orcs Must Die! and Dungeon Defenders each putting their own interesting spin on how it's played.
Sanctum takes a futuristic approach, and blends the strategy of tower defence with cooperative multiplayer first-person shooter gameplay. It's a blend that works extremely well: a simple interface between waves of enemies allows players to spend their limited resources on building towers, upgrading towers, or upgrading their own personal arsenal, then the enemy assault can be triggered at any time. Once the wave of enemies begins, players can navigate around and shoot the bad guys with a rapid-fire assault rifle, a high-powered but slow to reload sniper rifle, or a freeze cannon while their towers take care of the rest. Rather than having to run from one end of a level to the other, players also have access to an overview map, allowing them to easily teleport to various points in order to get the best vantage point on the advancing hordes. This also helps with planning tower placement, as players are free to block off certain routes when several are available.
Sanctum doesn't waste any time with storytelling -- it knows it's a fast-paced arcade action title and it plays to its strengths with a big, bold, clear interface, meaty sound effects, and an excellent trance soundtrack. But despite its arcade-y nature, it's a game that will take you a while to beat, too. Each of the four levels in single-player takes at least 45 minutes to clear, and then there are alternative difficulty levels, Endless mode (which features an extra map over the campaign) and the ludicrously challenging Turbo mode to take on -- not to mention the 4 player cooperative multiplayer mode.
Sanctum does a lot of things right and this keeps its gameplay flowing smoothly. Prior to a wave of enemies appearing, for example, it's possible to look them up in the in-game encyclopaedia to remind yourselves of their capabilities and weaknesses -- and enemies maintain a coherent visual style while looking distinct to one another, making them easy to identify in combat. The overview map facility helps what could have been a cumbersome building interface be highly strategic -- and in multiplayer, a whole new angle is put on this, as it's effectively possible for one player to play the game as a "commander" and order their teammates to put towers in specific places before taking up arms in the action phase.
In short, it's a very good, highly flexible game marred only slightly by an occasionally inconsistent difficulty curve -- and for some people, the lack of narrative progression might put them off progressing through the single-player campaign. Despite these minor issues, however, Sanctum remains a high quality game which the developer remains committed to -- and at the price, it's a complete bargain.