Low Prep City Design
What you can read under "Threats" on the campaign web site is basically most of what I know about my city. It's enough to run a whole campaign. I literally could ad lib everything from that, if I was willing to give up some of the sandboxy elements of play and switch back to my old school (circa 1988-1995) mode of play.
That older mode of play always started with a vision of a setting that was about as detailed as my initial write-up for City of Brass. The rest was created on-the-fly via stream-of-consciousness GMing. Riffing off what the players wanted to do, I might throw threats at them till one "stuck," then keep developing that.
In the sandbox mode, I prep a bit more. The encounter tables are created using that same riffing process, really. I want them to be sort of like Bangs in Sorcerer. "This thing just happened. What do you do about it?" (Even doing nothing about it says something about you, and propels elements of the setting forward.)
But let's look at what I did for the city design.
In West Marches setting design, there's probably some kind of distance-related thing going on where the nearby setting elements are low-level and the far-away setting elements are high-level. That way, 1st level PCs don't stumble into an ultra-high-level dragon encounter on the first adventure and TPK without warning. No fun.
So it's vaguely concentric circles, and the problems get bigger the further away from base you get.
In the City of Brass, the home base is the commune and the city is the wilderness. In West Marxist play, according to the rules in the "Marx & Monsters" write-up that inspired me, the city is crushingly dangerous. Everything out there wants to oppress you or take advantage of you somehow.
My city's concentric circles have a geographic component, too.
* the commune at the center
* the immediate neighborhood block
* the little neighborhood, Kickstone
* Ash Borough, ruled by an "elected" captain
* the two halves of the city, upper and lower, literally divided by a huge cliff face
* the entire city, ruled by an empress
* the wilderness around the city (a vast, rocky desert on one side, and a harsh sea on the other)
These geographic divisions have political divisions that go with them. Each of them is ruled by a bureaucracy that presses down on the ones below it. I've named some of the NPCs that control these and thought a little about their motivations, but they're all generally bad: greed, power, corruption, control.
There are other divisions, too, that are important. Social caste is a clear division, and the PCs are at the "center" of it, as in, the bottom:
* the poorest of the poor: squatters and other homeless (outcasts)
* the poorest laborers with houses or apartments, probably living in a place like Kickstone (lower lower class)
* crafters and other skilled workers who do repetitive jobs; also soldiers (upper lower class)
* artisans with more skill, controlled by the guilds; also lower priests (lower middle class)
* merchants and people with important skills; also upper priests (upper middle class)
* wealthy guild members and bankers who largely don't work anymore (lower upper class)
* minor nobility (middle upper class)
* major nobility; royalty (ruling class)
Each social layer presses down on the ones below it, too.
What this does for me, in prep, is limit what I have to think about. When I'm populating the random problems table with hooks, I don't have to concern myself with all of the city or all of the social divisions--only the ones the PCs are likely to interact with. That's more or less the ones nearby them.
For example, the Kickstone residents are getting angry at the commune because comrade Hengritte is selling their teens spirit potions (to fund her own habit). There's a group of Kickstone residents who get called "The Betters" who want the squatters / commune out of their neighborhood. Those are Kickstone problems.
Then there's the organized crime syndicate, the Blackguard. Even though that group is a city-wide thing, there's a very low-level Blackguard foot-soldier who collects Black Tax from the local businesses. So the PCs might deal with the 1st-level version of a 20th level threat.
The Masons Guild is lower-upper-class organization. The PCs cannot survive a direct confrontation with the Guild. Let's say its leader is 15th level. They are interested in the property in Kickstone and have sent 3rd level people to look into it. The PCs tried to pickpocket those surveyors last session and got their asses kicked by their guards.
By thinking in terms of these layers, I can project any organization down to the right threat level and add a plot hook that will draw the players into city intrigue that they won't be able to really deal with until they're high level (and maybe not even then).
They can deal with the local effects, but maybe not the global cause, if you follow.