A city coveted by many, but owned by none. (concept for review)

[Aristotle]

A city like this would become a hub of trade and negotiation between tribes. A great fair of tents would rise outside the city walls, as tribes come and go to trade at the city.

Great idea. The city is already home to the largest market in the world, but a second less official market established just outside might be fun. I'll have to include tent town on the map.

The guilds have been disbanded and free trade reigns, so most anyone can come and try to sell whatever it is they think they can make money on. Of course, space in the main market requires a license. These side-markets might skirt the rules and have no regulations. Shop outside the market at your own peril.

Just to throw out anotehr idea, instead of a small military, you could have a small civilian constabulary (or whatever), and then each of the merchant "factions" could have their own private "security forces". Disputes between factions could be handled in the "boardroom", so to speak, or could spill over into deadly street brawls between the "private armies" of the factions.

Absolutely! The senate runs the city, but mostly just to maintain it and mediate as needed. A police force of baton wielding constables keeps the peace. I considered a division of Inspectors that investigate crimes, but I'm not sure if that might feel too modern. I was toying with using it and D&D 4e to run a CSI style campaign in a fantasy city.

The merchant houses hold most of the real authority, having the influence to gain licenses to maintain large armed forces for the purpose of guarding caravans, warehouses, and so on... I'm definately interested in setting the trade houses up as corporations.