This is a Saberpunk setting. Take a stock D&D setting, drop it into a ginormous fantasy city, liberally add cyberpunk and postmodern tropes, and stir well. Add a dash of Ancient Rome for flavor.
Characters never leave the city, which is sort of like a giant, multilevel dungeon, only full of people. The city has been built up as well as out and it’s stacked in six layers. Below the topmost layers, buildings rarely see a sunbeam. In the lowest layers, terrible things prowl the streets, feeding on citizens.
People are forced to live in cramped conditions because the senate figured out that there’s a connection between people and magic. Magic doesn’t exist every in the world, only where people concentrate. And it seems that the more people you stuff together, and the tighter you pack them in, the more magic is created. Magic is useful, so the senate covets it. Magic is also chaotic. It transforms everything that it touches, and it is the reason there are a variety of races and classes in the city. Magic is power but it is also mutation.