Level codes (they might not all work):

JUNGLEWAR
BUCCANEER
HAMMERHEAD
DETONATE
AFTERSHOCK
BLITZKRIEG
CHESSPIECE
NOMANSLAND
BASTILLE
LIGHTNING
UNLEADED
GUNSRUS
CHAINMAIL
ONETWO
APACHELITE
EAGLEEYE
CROSSEYED
OLDSCHOOL
DIGIT
SURFIN
SHARK BAIT
MAD BOMBER
CHEESYPOOF
MPH
PHOENIX
AVENGER
WARRIOR

As already mentioned - you cant enter all of em 
as valid passwords - maybe they are used as some 
cheat code or whatever.