I think the achievements for the hidden levels are bugged. I got the one for completing 4 hidden levels when I finished my fifth one, and now I've done all the hidden levels but don't have the last award for it.
Love how you don't need a gun to use it, you can reload a gun while using another one, survivors don't need ammo and can't use flamethrowers, lasers, or tesla guns, and that somebody thought their instant death gun would be balanced by destroying their own defenses...