Mario's Castle is a mod for registered Wolfenstein, made by Mario Maniac. It was released on September 27, 2003.
The mod features 60 levels, with only E6L8 remaining from the original set. Many graphics have been changed, with some originating in Project: Eisenritter. Also featured are Nate Smith's blue guard and BJ Rowan's SS officer.
Level design is notoriously difficult at times, with practically all levels featuring only skill 1 guards - and often many.