Dis- everything you know

It's infinite in diameter, but it has an outside. Big green spikey mountains cover the layer of Dis. Those traveling toward the capital city, also called Dis, end up suddenly in the middle of the city without seeing a transition. It's confusing, but that's the way the baatezu like it.

The city constantly shifts and changes as the Archduke's petitioner-slaves tear down and rebuild it. Some say the burning iron walls respond to Dispater's will, moving as the lord desires it.

Although the streets and walls burn flesh, those with sufficiently thick boots or magical protection can survive it, and Dis is probably the most thriving center of cross-planar trade in all of Baator. Lawful and neutral fiends of all descriptions walk the streets, from lemures, nupperibos, and other damned soul-things to pit fiends and diabolic nobles. Many rakshasas live here, as well as tieflings, half-fiends, and humanoids of all races (elves, dwarves, hobgoblins, humans, and so on). Demons and celestials are not welcome unless they've turned toward Law and Evil. Mortals of any alignment are encouraged to come, however, in the hope that they will be corrupted.

It's said that Dispater has the ability to twist and manipulate the emotions of others, and it's said further that the Iron City of Dis has every pleasure and pain imaginable, every forbidden torment and delight available at a price, though the price may be far too dear for some. More erinyes and kocrachons are found here than anywhere else.

A few districts of Dis are described on the old Mimir site: http://geocities.com/athens/7117/chant/burg2.html#baator

Iron Fist is an administrative center, while Safe as Houses is a center of racketeering.

A Tiefling's Exultation mentions another district, the Tombs of the Heretics.