In the interests of full disclosure; This is a blatant kleptoverse.

We wanted to test out QAGS, and I’d had an idea for a police procedural sort of game, so we decided to do a one-shot and sort of make it up as we went along¹.
There are elements from Terry Pratchett’s Discworld, quite a lot of Sigil from the Planescape setting, and most likely some other stuff in there too. I know a part of the scenario came from an Ellis Peters novel, and I’m pretty sure another part is from a scenario I read, but I can’t recall where I read it.

The City is roughly circular and slightly bowl-shaped, sitting at the junction of two rivers which flow in through the upper gates, meet in front of the Guild of Burial, and flow out through the lower gate.

In many ways, it’s a standard walled city. There are rich & poor areas, shops, artisans, traders, mansions, and hovels. The sun rises and sets, summer is hot and winter is cold.

It’s the non-standard things which make The City unique.

There’s nothing outside the City Walls; It floats in a star-filled night which is never entirely banished by the sun.
Nobody knows what’s underneath The City. Some folks say it must be really nice, because nobody who climbs down the outside of the walls ever comes back, so they must want to stay there.

The City is filled with Doors which lead to other places.
Some you just walk through, some need a key or a totem, some require a song or a magic word or a drop of blood, and there a some which the Guild of Ways has no clue how to open.

The Dead Do Not Rest In The City.
Seriously, that’s what’s carved over the doors of the Guild of Burial, whose sole job is to ensure that nobody is buried (or maybe ‘entombed’, as there’s not a whole lot of loose soil in The City) within the walls.
They’ll collect the body, transport it home if they can, perform the funeral rites appropriate to the person as well as they’re able, and lay them to rest as respectfully as they can manage.
Most importantly, they lay them to rest outside of the city.

The City is divided into three quarters by the rivers.
Second Quarter sits between the upper branches (Sunrise and The Other One), and is where you’ll find the most important guilds, merchants, and people; That is to say, the rich ones.
First Quarter sits on the right bank as you look downstream – It’s a mass of small businesses, small workshops, and housing, usually in the classic “we live above the shop” format.
Fourth Quarter sits on the left bank, and is where any trader or merchant who can’t afford Second Quarter can be found.

Nobody knows why there’s no Third Quarter.
There are mentions of it in City records, artworks depicting the occasional City street scene which doesn’t exist, and some extremely long-lived² folks who recall living there, but no maps showing where it was, and no record of where it went.

¹ I was the one who decided that the forensic specialist should be a Beholder, one of the players named him Grackle, and while I’m not sure how he got a Welsh accent, we all decided he had to be called Grackle the Look from that point on.

² Or maybe just long-existing. This is, after all, a fantasy setting.