The Crawling City is a great metropolis of obsidian and ash. It moves across Gehenna, moving from Layer to layer at the will of the its master, the General of Gehenna. The Crawling City moves by virtue of thousands of fiendish, fire-immune legs grafted under the massive lower deck of the city, allowing the metropolis to cling to the steepest cliff face of Gehenna and slowly ford the broadest river of lava. The general is an Ultroloth whose power is said to approach that of a demideity. In theory, all yugoloths answer to the general, although the yugoloths' proclivity for plots and webs of deception means that many have divided loyalties.
The city has low barracks for Baatezu, Tanar'ri, and other assorted elite mercenaries, and siege towers housing potent war magic. There's also a war academy where brilliant Fiendish strategists teach their lore to up-and-coming officers bound for the Blood War, and massive factories where smithies constantly turn out the latest in fiendish military hardware. The Crawling City itself, in all the millennia of its existence, has never directly entered the Blood War. Ancient prophecies tell that should it ever do so, the Blood War will finally reach a decisive conclusion in an apocalyptic final battle.