Short version: A big surprise. If you like tower defense at all and don't mind FPS, give it a shot. A great blend of strategy + action.
Long version: Sanctum is great for TD fans that either like or don't mind some FPS action in their games. The two aspects blend together very well; on lower difficulties, one can pretty easily focus on one over the other and do fine, while on higher difficulties both are needed pretty equally. Additionally, there are towers that help your shots and a gun that helps your towers, ensuring that the two aren't just separate parts of the same game but actually integrated together.
Even aside from FPS tactics, there's a fair amount more strategy than in most TD games, as the player must build a map for the enemies to travel on and building good maps can be necessary for survival. Building a map that makes enemies take longer to travel necessarily means less space for defensive towers, balancing the game with a choice of longer path or more firepower. And of course, correct positioning of towers for the most effective use of each one is as important as always in a TD game. Even the size of certain towers can be a strategic point for ensuring the player can move around flexibly.
One can bring 3 different guns and up to 6 different towers to maps, ensuring many different ways to play the game, although the total number of guns and towers may be a bit low to take full advantage of that.
If there's one problem I have with the game it's that some of the map designs come off as very lazy, but you'll largely forget about that when constructing a maze. I haven't played much multiplayer or survival mode myself, but in later waves of longer games, guns can also come to be seemingly ineffectual, as damage doesn't scale at all with the growing power of enemies. Sanctum does what it does near perfectly and there are very few changes I would consider making to improve it.
Posted: November 18th, 2013