Whilst most of Vralkast is contained within large, giant-traversible caverns with constructed buildings, Unda is a maze of small tunnels, riddled with carved out chambers that the residents call home. It weaves in and around the other districts like worm burrows, simultaneously the biggest and the smallest district in the city. It's tunnels are cramped and small, perfect for the smallfolk who live there, and a harrowing crawl for everyone else. They often say that if you can walk into Unda, you belong there.
Unda is one of the most diverse places in the city, and its people have a very strong sense of community. Many of its residents consider their ward, or even the whole district, to be their family, and family sticks together. They prefer to sort out their own, among family, rather than involve outsiders in their affairs. Nobody outside of Unda knows the full extent of the district, nobody's ever mapped it, no one save its residents know how deep it truly goes.
Over time, migrants to the city have brought their own cultures, traditions and faiths, which have all blended together in the tunnels into an informal pantheon. Many of the tunnels have small shrines honouring their gods or philosophies, and there is no issue with these shrines sharing space with the gods of other faiths.