Arak Yor originally appeared in Hektula's Khyber Codex.
Arak Yor is a vast, ruined city built within a vast ravine deep in Khyber. The city is built up the canyon walls and is over a mile high. Ancient constructs roam its halls and streets, tending to their duties as if their masters aren't long dead. Arak Yor was a city of artifice, once, but now it's a hollow monument to the fall of a civilisation.
The center of the canyon is dominated by a central Spire, which rises from the base of the ravine at its widest point and rises above the canyon walls that hold the rest of the city.
Arak Yor was first settled by ancient dwarves for the effect it has on the mind, inspiring great works of artifice and innovation. There is a curious property to the demiplane, everyone within can understand all languages they hear or read, and concepts seem to cling to the mind in a way that makes learning far easier for most people. Information is just better retained within the walls of Arak Yor.