The 13th Floor is a map set for registered Wolfenstein, made by Matthew Barich. It was released on February 28, 2001.
6 levels are featured (one of these is a secret level), and all are played on Episode 1. Essentially a map set, the 13th Floor series was later issued with a different set of coding changes, though this map set also showcases some of the trickery Barich would later explore.
The 2001 version was simply a map set, and it is thus playable in SDL.
The 13th Floor: Special Edition
A special edition (SE) was released on June 12, 2005. Once again the six levels seen in the 2001 release were featured, but added was a bonus level at level slot 16. Furthermore, the source code was altered and a handful of new graphics also appeared.
This particular release was listed as being a beta.
The 13th Floor: Special Edition (Demo)
Roughly half a year later, on December 25, 2005, another version was released, labeled as being a demo rather than a beta. In addition to the maps already featured in the previous two versions, this version added two extra secret levels, and even more changes were made to the source code. These include the addition of six keys, trigger switches and separate ammo for each weapon.