It was the first time I played a "Hybrid" FPS/TD game and I have to say that I loved it.
+ Very decent graphics.
+ Every Turret and Weapon has upgrades.
+ You can build mazes to make the enemy take longer routes.
+ Full of "SECRETS" to discover (sometimes more than one per map).
+ All enemy have a "weakspot" ( this makes some turrets stronger for each kind of monster, also makes you aim at specific parts of the monsters).
+ Very challenging. Try the insane dificulty :O
- Sometimes it gets boring when maps take +45minutes to beat all waves or you have a nice Turret and maze combination built to do most of your work.
- Multiplayer has almost no players. (Never seen more than 7 players online).
- There is a buggy achievement. (Takes forever to get it, even though it's one of the easiest)
- You will probably need some guides to find all the secrets because they are extremely hard to find on your own. ( I used guides for most of them :$).
It gave me some good hours of fun and I can't wait to start playing Sanctum 2.
Remember this is just my opinion, some of my (+) can be a (-) to you and viceversa. It depends from player to player.
Also I'm sorry for my English. (I'm Portuguese).
Hope my review was helpful to you! :D