Got the game for free when Steam (or the Developers) randomly gave it away for free.
Community is small for this tower defense game. It is hard to find co-op games and achievements are fair and not tied to any co-op parts (which is amazing). 4 playable characters with a ton of gadgets and weapons. Levels are hard, even on Normal mode, you mostly need a co-op partner in order to win levels past level 3 or 4, it's challenging which what games the game pretty good in my opinion. Graphics are decent and have all the graphic settings you will normal see in an FPS game, even a FOV button.
Be warned though, the last update for this game was on April 25th 2016, so expect the game an 'as is product.' Don't expect any new updates, DLC, etc.
If it goes on sale, it's not a bad game to pick up and just play randomly with friends.
4.0