The oldest backrooms level is a very long house.
The longhouse was the first very large building designed by humans. Some of them were massive. So I propose a backrooms level that's just a long house that extends into infinity, maybe with partition doors at intervals. Dead fire-pits, dry sleeping spots.
Discarded carvings and broken cooking pots.
And always light flickering through the walls, the sound of running animals and chaos. #backrooms