Have a large basilica. Take the original St. Peter's or Trajan's Forum and market as visual examples. Large front gate with buildings that have the market offices. Just inside is a large open area that is rented out plot by plot, now filled with non-permanent stalls acting like a farmer's market, carrying food and other common goods. The major paths including the one from the gate to the covered shops on the other end being the busiest and most desired spots. The side areas are not rented out except day to day on a first come, first served basis. Here will be people with a blanket covered with the goods they wish to sell like flea market. There will also be old women with the collection of herbs, insects, and bones ready to make potions, holy men that will do blessings for alms; and other people looking to sell things that the normal shops will not buy, at least for the money the seller wants. IN the back is a larger multistoried building filled with shops that sell clothes, goods, brokers, and services that warrant a prestigious shop in the city market. At night everything is locked up and under guard.
Out to one side is a large collection of shops and stalls that sell goods after the market closes and all through the night. The entire block is not a single building, but more like a cluster of buildings that have all been built on top of each other. Inside are shops and even some residences laid out in no orderly pattern and a maze of corridors, ramps and stairs. Children wait for somebody to look confused so they can lead them to the shop they trying to get to for a few coppers. Here are the shops that do not need the prestige, security, or rules of the main market building. Everybody that works or lives here knows everybody else and recognise those who don't as such.
Is there something you want to buy. Somebody here either has it or a way to get it. Have something to sell, somebody will give you something for it. Have money? Somebody will convince you that there is something you want. Need money? Somebody here knows how you can make that money, perhaps by aquiring something somebody else wants.