I’ve been a Tower Defence nut for quite some time, and have spent many hours playing simple but fun TD variants such as this one. While I wouldn’t call myself a purist of the genre, I don’t really get into some of the modern takes on the genre,such as Plants vs. Zombies and so on.
So if you’d asked me a few weeks back whether I thought mixing Tower Defence with FPS would work, I’d probably have said “Nope”.
However in the case of Sanctum it works very well indeed.
Sanctum is a First Person Shooter Tower Defense where you take the role as Skye, an elite soldier sent out to protect her home town, Elysion One, from hordes of mysterious alien creatures.
To succeed you have to construct and upgrade defensive structures. What makes Sanctum unique from other Tower Defense games is that when the havoc starts, you jump right into the action with your own weapons and play a key role in the defense. Sanctum has taken the best features from First Person Shooters and Tower Defense games to create something totally unique.
That paragraph is a little misleading in that it tries to imply there’s a bit of a storyline. There isn’t - but that doesn’t detract from things too much. On top of your standard array of towers with varying abilities your character has 3 choices of weapon (assault, sniper, and freeze) – the weapons can be upgraded and come with a secondary fire option which adds a good range of variety. There’s a range of maps with some interesting terrain options, and there’s co-op multiplayer on top of the standard single player.
Sanctum is a pretty simple game, but if you like Tower Defence it’s well worth checking out – it’s a lot of fun, and at USD$15 (or for free if you have someone kind enough to gift it to you 
) it’s great value for money.