Ruins of a lost city
NPCs are sandworms, ritches, snakes, fire and multi-hued drakes, and horrors.
The top floor is desert theme, and may have vaults.
The second floor is cavern type. The schematic for Payload is on the second floor.
The third floor is special, and contains the Old Psi-Machine, which gives the quest Mystery of the Yetis, the replacement for escorts in the Embers of Rage campaign. The third floor has a fixed layout, and it has no NPCs, except Guardian Psi-Ghosts, which are not hostile to you. (Incidentally, they have stealth, so you may not see them.)