I have a fantasy city where the respectable areas are literally built on top of the slums - essentially, it's on a series of bridges or pillars, with lots of forgotten crawlspaces and hideyholes for my characters to exploit. I know this is, on its own, viable, if somewhat heightened from reality - Edinburgh had wide bridges that buildings were built up to, for instance, and there's lots of examples of large underground buildings being built and then abandoned.
The city has a big wall around it, rising up about to the level of the respectable areas, and a large river outside that. The wall is intended to protect the city from flood; the climax involves the citizens being trapped inside the city as it burns. (A shift in the river after the city was built on stilts made floodwaters rise much higher, necessitating building the wall to stop the foundations from washing away.) There is a large harbour, with a gate to prevent floodwater from entering. Due to its central location, the city has historically been a good hub for trade; it's actually the only viable industry it has left, the rest having become fronts for endemic corruption or organised crime over the last ten or twenty years. Magic is part of the history of the setting, but it's unknown in the time the story is set and if it was used in construction of the city, it would not be obvious or visible.
Is this plausible as a setting?
- Is the construction of the city plausible?
- Does the river shifting explanation for the wall make sense?
- Where would the stone for construction come from?
- Where would food come from, and would there be enough to support an obscenely wealthy upper class and a significant underclass?
- The city doesn't have canals. Is this implausible?
- What would need to be true for this to be a plausible setting?