Gehenna

Here is where I will place general lore notes on my specific version of Gehenna and the Crawling City. Anything that is not written here is assumed to cleave to the vision of Gehenna as presented in AD&D 2e's Planes of Conflict: Liber Malevolentiae and Faces of Evil: The Fiends and D&D 3.0's Manual of the Planes.

The Crawling City: Physical Properties
From outside of its protective force dome (specialized in repelling celestials), the Crawling City appears to be situated on a circular platform about 12 miles across, with approximately 9,000 fiendish, fire-immune legs grafted under its lower deck. Inside its dome, the Crawling City's extradimensional spatial nature is revealed, for its circular platform is actually 120 miles across (resulting in an area a little smaller than Belgium's area).

As a result of its extradimensional space, bags of holding, portable holes, and similar items do not function within the Crawling City. Fortunately, just outside of the city's outermost rim are many yugoloth stores selling (expensive, and, depending on one's haggling skills, overpriced) bags of storing, which operate by shrinking items down to incredibly minute sizes.

The Crawling City is a labyrinthine maze of spires, each several miles tall. Most are constructed of Gehennan obsidian held together with ash, but some are constructed of higher-end materials like mithral, adamantine, and/or glassteel. Wide and sturdy walkways wind through the towers, each teeming with a crowd of pedestrians and fiends teleporting back and forth. Generally, the higher one goes in the towers' tiers, the higher-end the floors will be.

The Crawling City: Districts
The Crawling City has a few dozen districts. The most notable ones are:
 * Fire Opal District: Upper tiers of the city's center. Named after the eyes of the many ultroloths who make their estates here, yet are dwarfed by the homes of unique yugoloth lords.
 * Fox Tail District: Upper tiers near the city's center, adjacent to the Fire Opal District. Named after the fluffiest type of arcanaloth's tail; the hound/jackal/fox-daemons run many magical, scholarly, and/or educational facilities here.
 * Silk Wing District: Upper tiers near the city's center, adjacent to the Fire Opal District. Named after the immigrant erinyes and succubi who work here (but not necessarily live here) in the many businesses of entertainment and worldly vice, catering to elite clientele.
 * Soul Gem District: Upper tiers near the city's center, adjacent to the Fire Opal District. Named after the currency used for personal-scale purchases in this high-end commercial district, which contains quite a few infernal department stores.
 * Larvae District: Middle tiers near the city's outskirts. Named after the currency used for bulk-scale purchases, which usually entails purchasing other kinds of larvae, Gehennan petitioners-turned-slaves, or Blood War armaments.
 * Chitin District: Lower tiers near the city's outskirts. Named after the rigid exoskeletons of the mezzoloths and the dergholoths, the insectile "commoners" of daemonkind, who own many homes in this residential district.
 * Pincer District: Lower tiers near the city's outskirts, adjacent to the Chitin District. Named after the crustacean-like pincers of the piscoloths, the sergeants of the daemonic armies, who whip weaker yugoloths into shape in this rank-and-file military district.

General of Gehenna
Nobody knows where the General of Gehenna lives, and nobody has ever seen his residence. Not even the Brigade of Darkness, the General's elite unit (led by the exceptional nycaloth Xell Dog-ears), knows where he lives.