This is a repost of a RPG.net post, in a thread for 'alternate New Crobuzons,' where you take three humanoid monsters and put them into the city as an oppressed underclass, and add three more monstrous monsters into the urban ecosystem.
A huge, winding sinkhole valley filled with multi-level tree-borne adobe spheres. The land around is arid savannah. Boolyut is an independent city, home to rich merchants who pay tribal nomads to escort caravans to other lands.
Myconids, the original inhabitants of the sinkhole and tenders to the trees. The humans came to the sinkhole first to trade with them, but eventually settled in the trees. Now, the human population dwarfs the myconids, but the myconids are still seen as the 'keepers' of the trees. Individual myconid works are barred from entering human dwellings without invitation, however, and as there is little to no truly 'public' spaces in Boolyut that leaves them vulnerable to violence. Sometimes, knots of myconid guards and human or kuo-toa muscle get into fights or even extended feuds at the behest of their merchant masters or colony sovereigns, or just over territory in the lower levels of the tree-structures.
At least five separate myconid colonies live in Boolyut. Humans know little of the how or why of political dealings between them.
Kuo-toa, once emissaries to the great treant tribes, have settled in the lower sections of the muddy tree-structures. They mostly bring fish and mushrooms from underground caves, or come to escape their brutal life underground, trading it for an equally brutal becoming hired muscle or mushroom-poachers in Boolyut. Both myconid colonies and humans hire out kuo-toa; kuo-toa without insignia of employment from either a myconid sovereign or human masters have no rights in Boolyut.
Zombies made of mud and the body parts of human and kuo-toa alike are the primary workforce of Boolyut. They work the mud-paddies and mushroom fields below, under the geas of the zombie-administrators. They only command the zombies; actually touching and assembling the zombies is left to rented myconid workers and shrouded untouchables.
The great dead trees that make up the core of the city's dwellings are the corpses of swamp-treants. Centuries ago, many of them strode across the landscape, but in the drying of the aboveground they retreated to the muddy sinkhole. In time, they entered a death-like senescence, and hundreds more from across the savannah joined them, making the sinkhole a sort of treant graveyard. Whether they can be rejuvenated and reawakened is unknown.
Ogres from the dry lands around come to trade, or are brought to be traded, as slaves. Ogre parts, as well, are known to make excellent raw materials for zombie labourers. Oni mages accompany these ogres in disguise, offering services to merchant masters or to trade for esoteric items.
Unknown to anyone but the myconids, an army of Clay Golems slumbers beneath the mud. They were constructed by the myconids to defend the treants; anyone suspected of harming the trees is abducted and dropped down into the mud. However, in the centuries since the golems' creation, the magic that keeps them quiescent and controlled has begun to wane, and at least one has had to be put down by myconid guards.