The tomb cities were an extensive network of crypts built beneath the capital of Endor to house Endorians who had lost their minds. Insanity was common among the Endorians, but because they could not die, they were simply locked away in these crypts after going mad.
Nimroth, Nimiane, and Nia also went insane and were locked away in the tomb cities. Nimiane and Nia were imprisoned together in one crypt, but Nimroth was confined by himself in the crypt directly beneath theirs. His crypt may have been on the lowest level.
The eighth cupboard leads to the crypt where Nimiane and Nia were trapped. Henry, Henrietta, and Zeke briefly visited this crypt in The Chestnut King when they went to Endor to search for Mordecai and Caleb. Nia was still trapped there, although Nimiane had escaped.