The map is surreal and medieval themed, making heavy use of the GSTONE textures. It is quite small but also fairly difficult with very limited ammunition and over 150 monsters. The architecture is complex, with many many structures on the floors and ceilings, as can be seen in the second screenshot.
The wad contains the MIDI “Ordinary Story” by In Flames.
Yesterday's Nightmare was originally supposed to be contributed to the Doom Wad Station community project, although the project was never finished.