What kind of policies, background elements and institutions would be required for a city to have 900,000 people in an Iron Age Setting?

Question

For example, Rome had between 600 000 and a million people at its height. This was due to urban planning, water aqueducts, trade and the grain dole. What would be required for a similar fictional city where almost no magic occurs?

I'm trying to make a fictional city where people live. The city is the capital of an empire with people coming from hundreds of areas, dozens of ethnicities and religions and a very powerful ruling class that makes policy.

However, the entire empire is currently at the Iron Age stage of technology.

Given this, how would such a city function?

Answer 1

Water
Could be a river or artificial canals or aqueducts. A river takes less maintenance, but it is more prone to producing floods. A good river also allows river ports for the delivery of goods. Consider something like the Nile, where the stream went one way and the prevailing winds went the other way, for relatively easy back-and-forth traffic.

Food
A large area providing tributes, however that is styled, or selling food. Rome was fed by grain from Egypt, even rice from India. Egypt was a province, India wasn't.

Building Ground
I mentioned floods. There are also earthquakes, avalances, snowstorms, and so on. The city should be in a place where they can expand rather than rebuild year from year.

But I think they key ingredient was:

Good Governance
All of the above need to be managed, even in the best of locations. The city needs a capable administration and military, century after century, to support a steady growth. Only the capital of a large Empire could grow that large.

Answer 2

If by "Iron Age Tech" you mean something like 15-5th century BC (say in Middle East), then reaching 900.000 would be massively difficult. Babylon is supposed to be the biggest city in the world in this period and it is estimated to be about 200.000.

You need massive amount of people working the fields to provide food to city dwellers and this in history required such metropolis to be center of power, usually capital of large empire. It would also need to be placed at navigable river flowing through very fertile land. It should also be near the coast as ships would need to transport the food from other large food producing areas (for Rome it was Egypt and former lands of Carthage). But that require reasonably good shipbuilding that may be impossible that early. You would also need to deliver massive amount of water which require good engineering, which again may be beyond Iron Age tech levels

Even taking all this into account, tech level may just not be high enough to allow for such city to survive.

That's why my suggestion would be to scale your city to known historical examples and replicate similar tech and enviroment (like Babylon - 200-300k). It is still big enough to be huge and to shock visitors from "ordinary" sized settlements. Or assume 900k is population of entire central region of the empire that should be dotted with settlements from small villages to large sized (20-50k) cities.

Answer 3

Policies and institutions

This is a bit tricky to answer objectively, without ideology seeping in. The authoritarian-minded will say you need an authoritarian state, and give their reasons and justifications. Democratically-minded folks will do the same.

The Indus Valley Civilisation has cities of up to 60,000 in the Copper Age, and all evidence suggests it had an egalitarian social structure: most houses are around the same size, and there are civil works like a system of roads and sewers, which must have been organised by an organisation which worked for the people's interests. A hundred years of excavating has revealed no signs of kings.

Background elements

A good level of rainfall and probably a river and/or coast.

A reliable food source, ideally something like a potato. A potato gives 10× as much calories per unit area as a grain. There is much discussion on how potatoes contributed to urbanisation. Doesn't have to be literally potatoes, could also be taro or anything like that.

A forest for growing wattles for building (plus all the other forest-resources). The Cucuteni-Trypillia culture had cities of 46,000 people in the Stone Age and housed them mostly in wattle-and-daub. Wattle-and-daub was universal around the world.

Control of the hinterlands

The city needs to be fed by the countryside. This load can be lightened a bit by fishing on the sea. If the same civilisation/country/trading bloc that controls the city controls the countryside for about 50km from their walls, that means about a hectare of farmland feeds every mouth in the city.

There will need to be reliable transport networks to get the food to the city. Roman roads would work, but river rafts and canals are easier. A horse can pull 30-50 times more on a canal's towpath than it can on a road.