Every major 3D shooter in the DOS era had secret levels. Wolfenstein 3D, DOOM, Heretic, Duke Nukem 3D, Quake – all had one per episode. Games that were not split to episodes (Spear of Destiny, DOOM II, Descent) had a couple as well; Hexen basically had one secret level per hub. It was like a mandatory element of the design. 🙂
