I want a big city in a tropical/subtropical climate, what conditons make it possible?

Question

Starting condition: I know how small cities form. It is mostly a trade center that requires an access to water and food. But some cities have more than 100 000+ people.

Angkor, Cambodia had apparently more than 1 million people. How is it possible for cities to grow so big? Is it just because they have access to more food?

• Climate: It is located under a subtropical or tropical climate according to this classification. Mostly: Af, Am, Aw/As, Cfa, Cwa and Cwb.
• Technology level: European middle age (late middle age to be more precise)
• Magic does not play an important role.

Result: A big city with several thousand people: 50 000-100 000 +

Process: How can I achieve a high population?

Since hot and humid climates receive more rain and can produce more food, is it a good assumption to say that large cities are more likely to form here than elsewhere ?

Answer 1

You'll need an impetus for forming a city rather than a scattered group of farming villages. Trade routes are good for this, particularly in places where there is a natural disruption of the route (ports, portages, etc.). Once you've got the initial incentive, the city will take on a life of its own: people will show up to provide services to the traders, then to the people serving the traders, manufacturing will be drawn by easy access to trade routes and a built-in market, which will in turn draw more trade routes, and so on -- growth will be driven by the presence of the city rather than by the original purpose of the city.

You'll need a water supply to keep the city's growth from being constrained at a small size: it's easy to supply a farm off rainwater, but a city needs a reliable, concentrated source such as a river. If your society has strong civil engineering skills and plenty of production surplus, you can import water via aqueduct/canal, but that's a very resource-intensive process.

You'll need a food supply, but it doesn't need to be nearby. Ancient Rome, for example, got much of its food from Egyptian farms.

You'll need peace, or at least superior military strength. Your city isn't self-sufficient, so you need to keep any combat far away from the city's supply lines. Nothing destroys a city like a famine.

Answer 2

There are several things that can, and historically have, contribute[d] to the growth of a city. Just off the top of my head:

• Defensibility against an enemy
• Along a trade route (oasis, river, port)
• Religion (sacred location that attracts pilgrims)
• Salt (really, any important natural resource, but salt, in particular, was an important item in early civilized trade)