The newly added 4-player co-op mode has now come out of beta and is live for all players.
http://steamcommunity.com/sharedfiles/filedetails/?id=635911740
Along with the addition of co-op, a new hard mode has also been included for players looking to make their trek up the tower more treacherous.
Here's some info regarding the nature of the co-op:
Q: Why is this local co-op only?
A: This was a crazy feature we added by toying around with the code which started out as something that the programmers did for fun. An online mode would require reprogramming an important part of the game and is not feasible. This isn't something that was meant to be a feature of the game.
Q: Will you ever add online?
A: No. This would require remaking a substantial part of the game's code. Local coop is much more simple.
Q: How does this affect the game's story / campaign / game mechanics?
A: The game wasn't designed or balanced with co-op in mind, so many things about the coop are just things that needed to be done in a specific way in order not to break the game entirely:
- Player 1 still controls the main camera.
- Other players get to play with the character that player 1 chooses.
- Player 1 chooses when you can aim down (due to the camera change)
- Etc.
There are several obvious limitations in the co-op's implementation (because the game was created strictly as a single-player experience), however the 4 player co-op is hilarious and extremely fun.