In such a city, it's my thought that the guilds would run the city as a defacto government of merchant princes or maybe left over nobility. In either case various factions have their own agendas and I would think that some sort of walled off sections for the various feuding families/guilds would be present despite lack of immediate exterior threats, sort of like enclaves. Also, despite its distance, a city with that much wealth would be highly coveted and always under a threat...even freemen will protect their homes from marauding tribes. So, I would think that this city would be walled nonetheless, taking into account the evolution of the city. On the other hand, a Great Fair (a la David Eddings) is a town of tents under the control, and policed by, one certain kingdom thus needing no walls. So those are 2 options that I see: walled city with walled enclaves and private armies of feuding factions or a great fair type of tent city already under the rule of a kingdom. To go out on a limb, it could be a thieves den or pirate city with slapdash and shoddy construction and the usual lawlessness, debauchery, and villainy with tunnels under the walls, secret entrances, bribed guards etc. that in reality remove the effectiveness of any security measures. Or lastly, it could be a nomad city...sort of a city on the move, depending on the season, where all of the buildings are actually wagons and they migrate up and down the coast.

My 2 cents