I think I'm inclined to pull a Dragon Age here. In Dragon Age, the furthest places seems to be roughly as far away as Rome is from London, or perhaps as Istanbul is from London. Roughly Antiva to Tevinter, or Antiva to Par Vollen. It's a ways, but not so far as to make trade impossible. Fenris is not especially old and he made his way from Seheron in the far north to Kirkwall, which is a few weeks by foot from the southernmost point to which anybody cares to travel, in not more than a few years (judging by his apparent age), stopping frequently for short periods and wandering without a set destination in mind. There may be other parts of the world, but they're so far away that they aren't really part of the discussion. Maybe there are a few colorful travelogues, the equivalents of Marco Polo and Xanthippus and Herodotus, that tell fabulous stories of distant countries that nobody takes particularly seriously.
What that tends to suggest is something the equivalent of, you guessed it, medieval Europe. Everybody shares a common religion, and though there are vast cultural differences there are certain underlying similarities, which would be, in this case, the lie, and presumably the religion to which it's attached.
I'm not sure I like that. There should definitely be something out there, somebody with different views of the catastrophe, places where forces are at work that aren't entirely understood by characters invested in the city's culture. The problem is that when you have too many of those you wind up playing this weird game of cultural counterpart. For instance, the city is clearly a cultural counterpart of Italy. It's a Renaissance city-state in a Renaissance/High Medieval country. I did that intentionally because a) it's familiar to me historically which makes it easy to crib ideas, and b) it allows me to play with elements that I personally enjoy, such as office-holding and factional politics. I don't want to have to come up with the counterpart of the French and Germans during the Renaissance, and then have "the Mongols" and "the ancient Chinese" and "fantasy Japan." That's dumb.
On the other hand, one of the things I don't like about the Mercedes Lackey novel I've been reading, Lark & Wren, is how woefully vague everything is in terms of geography, ethnography and sociology. They're in a kingdom called Rayden. There are other kingdoms, close enough that they're all actually united in one super-kingdom (ruled by a high king, so maybe it's a high kingdom? Who knows). The residents of Rayden know next to nothing about the neighboring kingdom, Birnam. None of the characters have heard any news from Birnam for the last five or so years, even though they are wandering minstrels who consider it a part of their job to spread news around the country. The last news anyone remembers they only heard because it personally impacted them in the job they held at the time. Most of the characters aren't aware that there was a bloody coup in Birnam five years ago, and none of the characters know anything - even rumor - about the old king or the new king. The characters are the leaders of a trade association of traveling minstrels, but they have no idea what life is like for minstrels on the other side. Even the Gypsies, notable in this setting for having a gossip-powered information network second to none, have no clue what to expect in Birnam. I mean, that's crazy! Birnam is a black hole, apparently! Going to Birnam should be like a someone from Soviet Russia coming to the United States - complete culture shock. In practice, it seems like crossing the border from New York to Pennsylvania. The characters speak the language, they seem to look live everyone else, their money is good, officials wave them through checkpoints, they know from past experience exactly what conditions will be like for servants at the royal palace - I mean, come on! When I go to a new deli, I'm never sure how to bus my food. It should be at least that different. There's no sense of nationality, of local custom, in Lark & Wren at all.
So I guess that's a dichotomy to not fall into. On the one hand, the fantasy counterpart culture. The reader goes, "Oh, this is Byzantium with magic, I see." Then they read 20 pages and decide, "Probably around 1400, too." On the other hand, Fantasy Country, which has no attributes except those incidental to the plot, and no regional variation (except those incidental to the plot).
Dragon Age has a case of a third option in the form of an alien society, which is neither an historical counterpart nor bland, the Qunari. The Book of the New Sun likewise - every culture in New Sun is pretty vastly alien, and doesn't always make a lot of sense to the reader. Gene Wolfe's accomplishment there is to keep you interested with faux-medieval tropes that you think you recognize, while actually representing something much stranger.
Which brings me all the way back to my opening questions. What's this world like?
Well. The area around the city is politically fractious. Travel is difficult, outsiders stand out. Everyone speaks the same language, but there are strong local accents, even if you live in a village a few miles a way. Further than that, you have a "culture group" of people with similar customs, the same language with a great deal of regional distinction, similar (though distinct) local politics, etc. At some point, there's a natural barrier - a mountain range, an ocean, a desert, a jungle, a hostile and xenophobic kingdom, a thick forest - and past that, nobody speak the same language and the customs are utterly different. Politics, too, may take a vastly different shape. The people look different; their skin is lighter or darker, everyone seems tall or short, their clothes are different, their swords are no good but the horses are the best you've ever seen - or something. But there are still things you can agree about. You have a trade language in common, if you're educated. The religion is still there, and the lie. Then you have thousands of miles of that, all honey-combed by mountain ranges, wide rivers, seas and oceans, etc. Then, eventually, you start to people like that living alongside stranger people - people with completely different skin, language that sounds utterly strange, writing you can't make out. And they don't believe in the religion, and they don't have the lie.
Going to need to flesh out some specifics, here, clearly. Let's set a goal of pinning down some things about:
- The city's "culture group."
- A nearby "culture group."
- A distant "culture group" that accepts the lie.
- A distant "culture group" that doesn't accept the lie.
- An alien "culture group" that inhabits the city - where is the city's Chinatown, and how have its residents preserved their culture? How have they lost it?