Thuldûmkarak, known in Common as the Deep Stronghold, was a Dark Dwarf city carved deep in the black stone of the Middledark. Nearly every tunnel leading into the city was filled with Faerzress, its strange glow warping light and disrupting magic, which made the approach dangerous to outsiders. Once a Dwarven bastion in the Underdark below Scyseti's Cut, it had been taken over by Dark Dwarf invaders more than a millennia before Common Unification.
The stronghold itself was lit by eternal flame Spellcraft and the fires of smelters and foundries; the latter's gears and chains turned in the open air to drive the city’s machines. Smoke and heat hung heavy everywhere in the city, and the cavern echoed with fire, steam, and the sound of hammers striking iron. The air in Thuldûmkarak was thick with smog, which often caused a harsh cough known as Karak-lung to those not accustomed to its poisonous atmosphere.
Most who lived in the city were dark dwarves, serious and grim, with little patience for outsiders, and a third of the population were slaves, most often Goblins, Dark Gnomes, Orcs, and other dwarves. Derro also lived within the city in significant numbers, though they were closely watched as second-class citizens. Thuldûmkarak traded widely in the Underdark, and Dark Elf merchants often visited. Foreigners were allowed into the docks district, but could not move freely through the rest of the city without a writ of passage.
Minecarts and track systems ran through every district, carrying ore from the mines to the forges. Tracks extended outward into distant caves, where duergar laborers dug for rare minerals. Although the city lay nearly five miles below the surface, the duergar had built a funicular-style cart that carried riders upward through a carved tunnel. This track ended at a natural dome of geothermal heat, where travelers sometimes rested. From there, routes led toward the surface, the nearest opening being in a foreclosed mine deep below Scyseti's Cut.