One of the most requested features for the excellent first-person tower defense game Sanctum was four player co-op. Today, Coffee Stain Studios is releasing an update today that will give fans exactly that.
Between this new co-op mode, two new maps, co-op building, and the ability to "ping" locations on the map, Sanctum has received a good amount of fresh content in the less-than-three months since it's been released. The gameplay dynamic changed a lot between the existing single-player and co-op modes when I last played it, so this should give you and your friends a new way to play through all of it once more.
If you were holding off on the game before, it's 33% 75% off during the Steam Summer sale -- for both a single copy and the new 4-pack -- so you really have no excuse anymore.