Fenrecz is the main campaign city for the game that I'm currently running. Geographically, it sits in the middle of the Viridian Cliffs, a long stretch of sheer wall caused by some unknown seismic shift or magical disaster far in the world's past. Here, a mighty river has carved a box canyon and a small settlement on the river banks has become a huge and complex city that takes up the whole of the box canyon, from wall to wall and from top to bottom. It's loosely intended to be evocative of Vienna/Budapest of Early Modern times and is ruled by a mysterious Duke and his imposing copper-masked guards. The land to the north is scattered duchys and small holdings, southward is a great swamp delta where lizardmen and human bandits prey on merchant shipping.
As you may have guessed, Fenrecz is heavily influenced by Vornheim, as I bought Zak S's supplement prior to designing it. As such, it's chaotic and heavily vertical -- not only are most of the buildings towers like you see in Vornheim, the entire city is build on top of older versions of itself, creating The Warrens, a labyrinth of causeways, aqueducts, old buildings and foundations that sits underneath the city proper, where the Duke's men dare not trespass and where the lawless of the city reside, although there are pockets of relative safety. There are also giant cisterns carved beneath the city, used mainly for eel-breeding (and, it is rumored, for the ready disposal of unwanted corpses). The main architectural feature of Fenrecz is a snake-like switchback road that criss-crosses the whole of the city, allowing trade goods to be transported to and from the docks that serve as the city's north and south borders (the southern docks are well outside the city walls and essentially serve as a second, smaller and far more dangerous hamlet).
As much as The Warrens serves as a (highly populated and self-policing) megadungeon of its own, there is a more canonical one as well, unnamed as of yet, that sits beneath the city itself. The PCs have just stumbled across one of the many entrances in their most recent adventure, having descended a stone shaft running down from a secret entrance in one of the city's older tombs, carved into the face of the cliff-side on the Western side of the city. There are numerous hints and adventure hooks scattered through Fenrecz that lead to this construction and multiple factions vying to uncover the various secrets and treasures that lie in its depths, which go far deeper than any could dare to guess.
So far traversing the city has involved copious amounts of rolls on Vornheim's various random tables, with the most useful being the random NPCs, as the party now has a network of contacts, including a gossipy hairdresser whose loose tongue both got the PCs in contact with a scholar who may have information that they'd dearly like to have and also with a locksmith with connections to the Thieves Guild, who thinks that the party has information that the Guild would dearly like to have.