This is indeed a great thread.
DarkAngel; you made yourself quite clear, I just didn't read your post carefully enough. Another example of me acting like a demented Kangaroo
If the city was a producer then the food issue does still arise if the hinterland (the area from which the city draws resources) has shrunk in any way or if there has been a reorganisation of wealth and a disruption of food supply. This would probably occur because once you base trade and supply on abstract wealth (coinage) and then take away the guarantor for the currency, all systems would collapse, at least temporarily.
The chaos caused by this would be very profound but could be partially alleviated, in your City, because of the presence of the old nobles. These would probably have seized power if their wealth is based on ownership of land and serfs as they could supply the cities food needs and have manpower. They could probably rob and steal with impunity. This is what happened in Roman Britain; the Iron Age tribal leaders were originally used as client kings but once Rome left, they reverted to the old ways again because Roman coinage could no longer be trusted. In this chaos, I think food and other supply systems would be far less effective and the population that could be supported would drop for a while.
The merchant class, whose wealth is all tied up in coins and not material goods, would instantly be in trouble and their houses looted; their mercernary guards would all desert or even join looting once it was clear they could not be paid.
In a historical setting, the land would become the principal focus again, as most people try to become self-sufficient and so there is a natural tendancy for Cities to become deserted as people move out to find farmland. In a fantasy setting, people might stay in Cities to get protection from monsters as the walls would provide protection.
I think by 70 years after such a disaster, up to 60-80% of a major City would probably be abandoned, with the remaining population huddling together behind crumbling walls. I expect that such a settlement would be stable because the old landowners could have managed some sort of transition of power and chosen a leader, although the general collapse or order would encourage constant infighting between the nobles. I would also suggest that the abandoned parts of the city might well have been demolished to allow for crops to be grown inside the city walls. As for the rest, life would settle down to some semblance of order, and the Noble leader would probably start minting his own coins. Once this happens then abstract wealth again becomes possible and much of civilisation would probably return to the City if the whole cycle happened within about 10-15 years of the fall of the Empire.
I would also suspect that animals would be brought within the walls at night and that much of the population of the city works outside the walls herding the animals or growing crops by day.
Hence such a City could have quiet a high population (perhaps 30% of the original Imperial population but possibly with much greater crowding because of refugees from outlying settlements having crammed in) but the city would be a very noisy, smelly animal dung plastered place that was very agarian in character with the old crumbling civic buildings of the Empire slowly decaying away.
I would also suggest that more fortifications would surround the city, in the form of earth ramparts and ditches to protect the farmland immediately abutting the city.
The final element of such a city would be crime. Chaos creates the opportunity for crime lords and your city is Certain to have one. Especially as the nobles would be sure to exploit the common people for their own gain. Also, expect that most people in such a city would be serfs and beholden to a Noble for a time and this might still be reflected in the makeup of society, even a considerable time later (perhaps it evolves into rival political parties or factions).