As a person with a fondness for the many "punk" genres (steampunk, dieselpunk, cyberpunk, etc), there exists a commonality among most of them: somewhere, at some time, you will encounter a tiered city. The lower tiers will always be full of factories, smog, poor people, and crime. The levels get increasingly better off the higher you go, up to the aristocracy looking down their noses at the city below at the top. Retro-future/raygun gothic settings are just about the only place to feature tiered cities as a nice place to live if you aren't in the top third.
Effectively, the city is a giant arcology. But what's never covered, amidst the multiple tiers of streets and sidewalks and such, is how it stays up. There's almost never any clear organization given, no order, no thought put into how it would actually WORK. Is the city just made out of big skyscrapers, with roads and sidewalks bolted onto them at various levels? Or is the city built with giant skeletal building frames, each city block having multiple levels, which are like any normal city block of the relevant class level, other than having other city blocks above and/or below them?
The tiered city gives an easy means of letting players know what kind of neighborhood they're in, but it's hard to use. You HAVE to decide exactly how it works beforehand. How do you get from layer to layer. If it's built just as a series of skyscrapers, can you get from one tier to another from within the building, or is there no access between tiers outside of designated points (or some manner security system, passcodes, etc.). Tiered roads are assumed to be for cars, where do you park?
As much as I like the concept and the possibilities it offers, it's something I need to sit down and sketch and ponder on, until I can find a "workable" means of implementing it. I first realized the need to sit down and figure out details in my latest campaign, a dieselpunk horror in an even worse version of 1930s Chicago called Lockport.