The nazi zombie levels are a bonus co-op mode where endless waves of the undead attack you and up to 3 other players in this fun survival mode.
Levels
Nacht Der Untoten – Night of the Undead
Verrück – Zombie Asylum
Shi No Numa – Zombie Swamp
Der Riese – Zombie Factory
Secrets
Hidden Easter Eggs in the zombie maps.
Tactics
Survival Guide – Tips and tactics to help last longer!