Mentioned in Equal Rites, the ancient walled city of Zemphis has a lively traders' market. The city is located located on the river Ankh where three overland trade routes meet. Due to the amount of trade passing through the town is built around a large central square which is part exotic traffic jam and part tent city. This town is where Eskarina Smith parted company with the Zoons after being forced to tell the Tribal Liar a half-truth about the business over the spircles. (This happened at the assayers' office on Pine Street). She sought refuge with an overland caravan, departing for Ankh-Morpork via the Paps of Scilla.
Zemphis is revisited in Raising Steam and is described, by Samuel Vimes, as a behavioural sink in dire need of Law. Vimes is looking forward to the day when Vetinari decides the town should be brought under Ankh-Morporkian law, freeing him to go there and legitimately interfere. For now, he is content to note this as a future possibility, as he has more important things to prioritise first.
There is more information about Zemphis in Mrs Bradshaw's Handbook. mrs Georgina Bradshaw emphasises that the Souk of Downsized Abbey is to be visited expressly at one's own risk. The smell of Klatchian tobacco of various sorts predominates, battling for primacy with that of strong goats' cheese. Mrs Bradshaw also notes that the writ of the Beggars' Guild does not apply here, and the city is a haven for aggressive unlicenced begging; it offers asylum to those who have broken the Guild-law and customs of the Beggars Guild and as such is home to many refugees from Ankh-Morpork.
In fact, the only part of Zemphis where anything remotely like Ankh-Morpork law remotely applies is its railway station. Station staff have the accepted right to chase beggars and un-desirable pedlars off the platforms and concourse. And trains and stations are now patrolled by the latest arm of the Ankh-Morpork City Watch, the Railway Police, who defend the interests of the company and the City on all trains, tracks and stations. Thus fulfilling Sam Vimes' desire to bring some sort of law here, and, no doubt coincidentally, extending the writ of Lord Vetinari and City law over hundreds more square miles and lots of valuable real estate. As well as all those small towns and cities nominally outside Ankh-Morpork's current borders, with either rudimentary or no jurisprudence of their own, but which have expressed at least tacit willingness to accept Ankh-Morpork law as colourfully and effectively embodied in Sam Vimes.
She also recommends visiting the Zemphis Falls as antidote to the City.