So Sanctum just came out. It's an FPS/Tower defence, and its looking pretty good. I got it myself and been having fun with it. There were some bug problems on release but most have been fixed and getting co-op to work as I understand takes a little effort with portforwarding until it to is resolved but still nontheless a fun game worth getting now.
The game is revolved around you (in first-person) moving around a set map with an open grid placing blocks and towers to combat enemy waves as well as shooting them with your own weapons. While normal mode can be beaten within some hours, the game adds a challenging harder mode and as well as co-op. The devs are also planning to add more content such as maps, enemies, 4 player co-op, and weapons in future patches. Hopefully some map editors to
A link below adds a good review I think to this game.
This game is currently 15 dollers and is on Steam