The countries in my D&D campaign world are decaying collections of squabbling bandit nobles, not centralized nations like Louis XIV's France. My dungeons are mostly divided among separate groups, not well patrolled jails or fortresses like Louis XIV's Bastille. But for some reason, my cities tend to resemble Louis XIV's Paris - prone to the occasional rebellion or riot perhaps, but generally recognizing a single civic government, paying taxes, obeying watchmen, and trying to punish breakers of the peace in an orderly way. It's like, when it comes to city design, I didn't get the 4e points of light memo, and I certainly didn't pay attention to the OD&D city-based random monster charts.
What would it mean to redesign my cities to match the post-apocalyptic power-vacuum assumptions of the rest of the setting?
1) gangs and monsters rule neighborhoods. A typical dungeon level looks something like this: "The death cultists are in this part of the level, The Eye of Fear and Flame is in this room, the giants control this area, and these rooms and corridors are empty and patrolled by wandering monsters." Let's make cities look the same way. A gang, an elected official, a hereditary noble, and a monster might rule four neighborhoods in the same city, separated by a no-man's land inhabited by beggars, kobolds, and PCs. One consequence: it's important which city gate you enter. The north gate is held by a different power than the south gate.
2) there's safety in your neighborhood. Cities can't be infinitely dangerous: they contain thousands of citizens who survive year to year. If you pay taxes to your local gang or monster, you might live in relative safety. When PCs first stay in a neighborhood, they might be approached by a representative of the local gang leader and asked for some token of fealty: some shakedown money or the completion of a little task. More than anything, the local ruler just wants to make sure that the PCs are not going to be disruptive. Once the PCs have paid their dues, they can live in the neighborhood without further molestation - they might even be able to call in a favor.
3) rival neighborhoods are dangerous. In an orderly city, the pace of urban adventures is often up to the pcs: barring time pressure, they can usually go home and rest whenever they like. In a "street crawl", unless they're in their home neighborhood, they're basically dungeon crawling through The Warriors. If you're mapping a city, you could treat a neighborhood like a keyed dungeon room: it's a guaranteed encounter.
4) Random encounters take place in the no-man's land. Whenever the PCs travel from one neighborhood to another, they pass through contested territory. You could treat this no-man's land like an empty dungeon corridor: make a random encounter check (6 on a d6). Typical encounters: thugs or guards from one or the other of the neighborhoods; low-level gangs or monsters trying to carve out their own territory; starving or desperate people driven to violence; toll collectors from one side or the other.
5) alliances shift. How can a vampire openly rule a neighborhood in a human city? If her lair in the South Quarter is sufficiently impregnable, she can laugh at the threats of the General in the East Quarter. If the Bishop of Pelor, who rules the South Quarter, sends clerics to attack her, she might enlist the General's help: surely he wouldn't care to be surrounded by a puritanical theocracy to the north and south.
6) there are exceptions. There are some dungeons, like the Steading of the Hill Giant Chief, in which all the monsters are of the same faction. There are others, like the Tomb of Horrors, with lots of traps and an absent or hidden bad guy. Similarly, some cities do bow to a centralized authority, and some have no real governance at all beyond some capricious magical punishments for rulebreakers.
OK, here is a tool for generating post-apocalyptic cities.
Each settlement is composed of some number of neighborhoods (1 for village, 1d4 for town, 1d6 for city, 1d10 for megacity)
For each neighborhood, roll on the ruler chart. The first ruler you roll is the most powerful, followed by the second, etc. if you want to know how any two neighborhood leaders get along, make a reaction roll (2d6, higher is friendlier).
Ruler chart: roll d10
1 remnant of the decaying order (knight, baron, duke, king, beaurocracy, rich landowner families)
2 duly elected officials (elder, sheriff, mayor, oligarchy, trade guild, independent neighborhood militia)
3 cleric (priest, bishop, cult leader, paladin, monastery)
4 rogue (thug, godfather, imposter, thieves guild)
5 wizard (archmage, necromancer, institute of learning or research, bard, elementalist whose magic alters neighborhood)
6 fighter (gang leader, general, warlord, humanoid tribe leader)
7 magical effect (prevents entering/leaving under certain conditions, zaps people who break certain arbitrary laws, is a giant game board, slowly transmogrifies inhabitants)
8 lawless slum (weak humanoid tribes, small human gangs, kenku, low level undead, fungus, oozes, diseased, exiles, multiple random monsters, rebel forces, battle ground between two neighborhoods)
9 boss monster who demands sacrifices (dragon, intelligent undead, mind flayer, lycanthrope, fiend, elemental, hag)
10 theme gang (all share a certain characteristic: costume, exotic weapon, race, age, bizarre slang, strange drug use, fearsome magical mutation, magic power)
Example: My city has (d6) 2 neighborhoods so I'll roll d10 twice: (2, elected officials and 1, old order). Reaction roll: 5 (slightly hostile). I'll say the most powerful faction, the elected officials, is a sort of neighborhood watch that arose from the city's vast slum. A young paladin, Sister Bridey, inspired gang leaders and merchants to unite and resist the abuses of the king. The weaker faction, the nobility, is afraid to venture out of the rich quarter. This state of affairs can't last long, as Bridey's rowdy allies are clamoring to loot and punish, and the nobles are looking for agents to kidnap or assassinate Bridey and fragment the slum alliance.