Adventures in an Unnamed City 1

So I have started my working on my urban fantasy campaign setting in earnest (wherever earnest may be). It feels a bit daunting, but deciding to use Constantinople (c. 1150) as my point of departure helps considerably. I have a decent collection of books on Byzantium, plus I just picked up the GURPS PDF on Constantinople. Starting with Constantinople has at least moved me out of the starting blocks.

My original thought was to use Constantinople juiced up with a bit of magic and weirdness. While I still might do that, I am now thinking of creating a fictional city with a lot of elements lifted from Constantinople. I am also thinking that I want a setting that I can use with multiple rule sets--GURPS, FATE, and Swords & Wizardry--so for now I am mainly thinking about setting and not system.

What has worked well when you have run a medieval urban fantasy RPG setting?