Dis

From Fiendish Codex II: Tyrants of the Nine Hells (3.5e):

Dis, the second layer of Baator, is taken up almost entirely by the city of the same name. Blackened, sizzling, and confusing to the senses, the city of Dis sits unevenly in a vale ringed by jagged mountains.

In keeping with Baator's lawful nature, the realms within its boundaries sometimes appear impossibly large. This situation is not the case in Dis—perhaps because the city embodies a paradox. Although it contains potentially unlimited space, those who travel within it always feel hemmed in, trapped, and oppressed. In fact, one can walk its scalding streets forever and never get anywhere.

The approach to Dis presages its spatial peculiarities. The traveler moves on a punishing slant from a ring of spiny mountains. A road of broken skulls winds toward the black walls of the distant city, and one can reach it only by following this macabre track. Eventually, the skulls transform into spurs of hot iron.

No matter how long it takes to reach Dis, the entrance always comes as a sudden break in reality. The walls loom larger and larger, then suddenly the traveler has moved past them and is surrounded by ominously looming structures amid mazelike streets. No two maps of Dis are the same because its configuration invariably changes by the time the cartographer finishes his sketch.

Building crews of least devils sweat and toil, tearing down old structures and erecting new ones with impressive speed. However, the improvements they make are never discernible, because the cityscape alters faster than any laborers could ever arrange. Supposedly, both the mundane and the magical alterations reflect the inner workings of Dispater's mind. His recent paranoia can be seen in the increasingly cramped, warrenlike nature of the city's new streets. Scrying devices have recently become omnipresent, so the walls of Dis have ears. Iron statues of Dispater follow passersby with red, paranoid eyes.

From Manual of the Planes (3e):

The second layer of the Nine Hells is a burning city of iron. The red glow of infernal heat burns within the scorched iron walls, and a pall of smoke rises up to form a general dark haze over the entire layer. For the city is the layer and the layer is the city, Dis. Some claim that a traveler who walks far enough can leave the city behind, concealed within a great ring of mountains.

Ranks of red, glowing buildings extend to every horizon, rising and falling according to the hidden terrain. Palatial mansions of particularly important devils and officers of the Blood War break the cityscape here and there. Every iron wall glows with heat, and casual contact with the walls deals 1d6 points of fire damage.

Even the iron cobblestones glow with heat. Without iron-shod boots, visitors soon writhe and burn in the streets. Screaming petitioners, captives from the Blood War, and mortals kidnapped from the Material Plane fill subterranean prisons beneath the streets. The sound of their agonized lament is usually audible from small vents in the city walls.

Some sections of the city host markets and bazaars where creatures from a variety of different planes meet. Most seek to buy or sell questionable wares, or hire a bloodthirsty band of mercenaries for work that only a devil could love.

Crowds fill the streets. Parades of devilish nobility attended by lemures and horribly sculpted soul shells vie with countless work gangs who constantly sally forth to repair, extend, or remodel the city at the behest of Dispater, Lord of the Second.