My game is historical, set in Massalia (modern Marseille) in 300BC. I'm trying to model the place using the usual rules, but I seem to keep butting up against hard-coded assumptions.
Massalia is a city-state. It doesn't actually control much territory beyond the city and it's chora which never really extended beyond a five-mile radius of the walls (it was completely encircled by a stone wall, the interior about fifty hectares). It had a massive presence in the coastal trade in the western Mediterranean (having wrested pre-eminence in trade flows into and out of Gallia (France) from the Etruscans) and deep into Gallia via the Rhodanos (Rhone) river. It was rich, but not because of land, but trade.
That map actually over-estimates the area of control Massalia really had.
It set up trading posts all along the southern coast of Gallia and northwestern coast of Iberia (Spain), places for ships bound for, and heading out from Massalia to stop, take on food and water, shelter from bad weather. None is bigger than a village in domain terms (they include Monoikos, Nikaia, Antipolis, Olbia, Tauroeis, Agathe). They were allied with other settlements set up by their parent city, including Emporion (Empuries) in Iberia, Alalia on Kyros (Corsica) and Elea in Italia. But all of those are technically beyond the sea-trade range of a Market class IV which it's size merits.
It might be a Duchy, from the largest settlement size (Massalia is 1200 families at this time), except it controls virtually no territory besides the city itself. It has trade links with two important settlements inland, Rhodanosia and Thelina/Arelate, but it ceased to have any effective political control over them a century earlier. It does, however, have economic control over them.
It's a major market nexus, for trade going into Gallia, and trade coming out of Gallia, but isn't really much of a destination in and of itself. They exported their own products; local wine, salted pork and fish, aromatic and medicinal plants, coral and cork, salt, olive oil, cups, mixing bowls, to inland markets in Gallia. They were a destination for re-export primarily of grain, amber, tin and slaves.
That control over a huge portion of trade should really merit a bigger market than class IV, even though it's a large town at best in population terms. Because it can't support itself from the land it controls (barely more than a few 6-mile hexes immediately adjoining the city), it supports itself from trade.
It also has no "prince" nor "ruling family"; it's an aristocratic republic ruled by an assembly of 600 citizens, with an executive body of fifteen elected from that number. And again, without trade, few of those citizens would have much wealth at all.
So how do I model this?