Sorry to comment spam, this is my last one. There should be an in game way to chat with the other players. Or at least some chat macros. (press 6=I Need Backup! 8=This Sides Clear, etc)
Multiplayer movement/controls kinda messes up after wave 2 though. Still a great time killer. Ah, never shot zombies to smooth jazz before. Somehow...relaxing. XD
Clever puzzles, wonderful ending. Perhaps there should be a Gateway: Aftermath? Find the fate/future of Laura? Find out who the grey man (player) is? And for that matter, why the girl had a plug in the side of her neck? =D