The Rolling City and the Devil Sun

This post is in response to a challenge from Anne at DIY & Dragons based on this post a the Githyanki Diaspora from 2009 suggesting an easy way to "Make Your Own New Crobuzon."

The Last City
Clacking, rumbling, the city moves. It rolls through the night on sixteen indestructible rails carved from the bones of dead gods. The shanties on its ziggurat steps rattle; it's bristle of towers sways. The city never stops for long, and it always stays ahead of the dawn. It's being chased by a vengeful god, the Sun.

The Devil Sun
There is a face in the green Sun, and it looks down on the world it hates with grinning, idiot malice. It chases the city across the face of the blighted world, through the ruined cities of the elder days. Where its morning light shines, its energy creates cancer jungles and fleshy masses of monsters. Even these wither and die under the force of its noon regard, leaving only blasted desert in the dying light of evening. The Devil Sun would destroy the clanking redoubt of the city, too, but it moves too slowly across the sky to catch it. For now.

Three Minor Humanoid Races
Xixchil once had their own city, but it was lost, and they bought their passage on the last city with their art. It was the Xixchil surgeons that developed the Warforged. The Xixchil are mistrusted because they live in enclaves of their own and practice secret rituals they do not allow others to see.

The Warforged were made to be the city's soldiers. There are many fewer now than there once were. They are officially accorded respect for their service, but many former refugees blame them for the loss of their old homes.

Athasian aarakocra live in the precarious high towers of the city. They are scouts and foragers.

Three Monsters
Clockwork automata serve in every level of the city, particularly performing jobs around the engines or on the city's undercarriage where living things can't go. Some damaged automata become rampaging clockwork horrors.

Obliviax is cultivated in some labs in the city for it's various memory uses: to fashion an anti-senility drug, to steal memories, or simply to make people forget. It has escaped and grows wild in some lower levels.

Arcane oozes sometimes crawl up the cities exterior. The gorge themselves to a torpor on the divine magic that powers the city. Sometimes they become a hazard and must be removed.