nulvl1 is a mapset for the registered version of Wolfenstein 3D. It was released in early 1995 (!) by an unknown author and offers a single new map along with a few inconsequential modifications to some Episode 1 levels. Does not have the MAPHEAD file, which you are supposed to recreate or otherwise summon out of the abyss of dark magick yourself.
- nulvl1 on Dropbox