Megadungeons of the Kingdom of the Wheel. After all, even if they were warped by Chaos Magic, there should be some expectation of more orderly remnants or elements of the original structure somewhere.
What might these be?
Corridors and Stairwells
If it was meant to be a fully functional underground facilities, there would be long (perhaps even wide) corridors and hallways for people to travel through. They would be designed to fit not just people, but perhaps even pack animals and carts -- particularly if the rationale was to store food and other supplies for times of hardship and famine.
Food Storage and Consumption
If there are people who are meant to oversee things like food storage, or two provide security (in case something from the interconnected Underground comes through) then there should be supplies and places to consume them in key locations. Therefore, food storage, food preparation, and food consumption areas should be found somewhere in the dungeons.
Bottlenecks and Saferooms
If they were paranoid, there'd be certain bottleneck areas that people could conceivably hold off armies from. And there'd be places for non-combatants to hide (secret doors with locks) in case they are overrun and need to wait for rescue.
Rest and Recreation
Places to sleep, places to congregate and have fun, places to practice or drill in their specific skills are likely. So, several areas where people can sleep (with larger spaces for the more important members of the community), and perhaps a general purpose empty room or two where things can be brought out of storage and used -- for an indoor game or two? Or perhaps even some military drills?
Maybe sources of water? A library? Something that keeps any records or logbooks of the people stationed below?
Last but not least: the Spires and Teleportation
Of course, one of the main things adventurers are looking for are the teleportation rooms (small and large) tied to the primary usage of the spires. These things were meant to move not just men, but materials, supplies, and trade goods across the Kingdom. There'd be at least one area meant for military use, and another area meant for public use (much like a teleportation port area).