The city is ruled by the Auction House King, who controls the entrances to the caves. He buys monster carcasses and other labyrinth goods. He then turns around and sells them to the Butcher Barons, who do the hard work of turing various rotting corpses and assorted dungeon goo into salable products. They've gotten good at this; Halstruk would be the world capital of alchemy if the Butcher Barons pooled their knowledge. Instead, they hoard trade secrets and seek to undermine each other. The Auction House King is an expert at playing them against each other; this is how he stays in power.
Adventuring is also a cutthroat profession. Hunter clans guard their maps and secret knowledge, and rivalries can turn violent. There are no laws in the caves, and any creature killed in there is fair game to be sold at the auction house. Good adventures enjoy wealth and fame. Average adventures scrape by. Mediocre adventures die.
The labyrinth is split into stratums, each with its own ecosystem, monsters and rewards.
The first stratum is know as The Emerald Falls. It is a series of mostly vertical tunnels over grown with a dark forrest. A small amount of light is brought in by a system of ancient mirrors.
At the bottom of the shafts there is what resembles a vast city for miniature people. This is the Fairy Hives. Fairies are vicious insects with uncanny intellect. They spend as much time fighting each other as they do fighting intruders.
Below the Hives are The Weeping Tunnels, so named because of the foul smelling muck that oozes from the walls. The tunnels are soft, wet and prone to collapse. They are constantly being dug out by giant worms and other vermin. There are strange things to be found in the rotting soil
Even further down, there is a strange warm sea of pink fluid. Know as The Mother Ocean, its waters are actually breathable. Bizarre half-formed creatures make their home here, the most fearsome of them being the unborn gods, powerful and mad psychic beings. Gods' brains is the most most expansive delicacy sold at Halstruck, only kings and emperors can afford it. There are rumors of a realm even further down, but no credible witness has ever seen it