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I am envisioning a trading port city/town, that occupies an island in a river mouth/estuary. Bridges link the island to the mainland.

The island is a mostly submerged, extinct volcano; and the crater forms the walls around the harbour and town.

The town is the major trading hub of the continent, and headquarters for the major merchant companies. Ships, wagon trains, and riverboats carry goods to and from all parts of the known world. There is some passenger traffic, but mostly goods.

How big would the town be, both physically and population? How big would the harbour itself be?

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It really depends. If it's a big and rich city located in fertile lands in subtropical climate (climate of eastern China) , it could be around 250 000 - 500 000 and almost double the number if it's the capital. Capitals usually come with more military personnel, aristocrats, bureaucrats, and a lot of resources are invested in the city, public buildings, temples... so it add a lot of people into the city.

Example would include Hangzhou, Nanjing. Inland trade is almost as important as far away trade. Cities can be linked by rivers or canal. Even a city far from the coast like Kaifeng had a very large population.

If the climate is more temperate, then the numbers should be lowered. London had somewhere between 25 000 and 100 000 people with huge population swings due to the black death. Vienna was one of the most important city at the time due to it's geographic position.

One important factor in the maximum size of the city will be the wealth of it's people, or the country in general. If they are really rich like Venice, they can import food. They don't need a lot of land to support a large population. If thy have a large empire like China or the Romans, they can concentrate the resources in the capital. Both Rome and Xi'an had a population of over 1 million people at their zenith. Both depended on food importation from other regions, else the local population would have starved.

Lastly, peace and stability are very important. That is another reason why China had so many large cities. It allowed them to build infrastructure like canals, to develop trade and make babies (it take some time to repopulate). On the opposite, Irak ceased to be a powerful country after the Mongol invasion. Many people died and the irrigation system became unusable. The country never recovered it's former dominant position in the region.

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Medieval technology means goods will be loaded and unloaded by manual labor. Perhaps a few cranes powered by oxen, but by and large people. So if the town is the trading hub for a continent, you need a lot of stevedores. These workers will have families, they will need services (food, clothing, shelter), and so on.

Without telecommunications, you will need merchants and clerks living in the city along with the stevedores. Again they have families, and again they need goods and services (more and better than the stevedores).

When I hear port trading city I think of Hamburg. "Hub of a continent" might be a bit excessive, but they were a major force in the Hanseatic League. Population in the five figures. Hamburg didn't have those crater walls, but it was protected by decent city walls.

Another data point could be Venice with a population in the low six figures. They were a hub in the Med.

Rome might be a bad data point because it was a capital of an empire as well as a trading hub.

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I'm going to push back a little at the numbers I'm seeing and say as few as 5,000. @Vincent's answer really opened up with the right answer - this depends on other things.

This may sound absurd, but count the number of ships you can dock at once. Then using the largest ships you could have docked, draw a walking path from the deepest hold in the vessel to the merchant's holding areas. Place one person about every 5 feet. This is how many people it takes to unload or load the ships at any one time, in the worst case. Certain kinds of loading requires not having that line of people there.

How many people live in this place is largely going to depend on two things- how many it needs, and how many actually want to live there. You said it was on an island at a river mouth. I could see that getting overcrowded, smelly, loud and generally unpleasant - if you wrote it that way, of course.

5,000 people is a lot to have in a small place, and depending on what they're doing, more is not always a great thing - they tend to end up working slower if they have to work on top of each other.

The point is, the fact that a place is important economically and strategically does not necessarily lead to it being popular. Other factors go into this, and a lot of it has to do with the personality of the persons in charge, and how that interacts with other people around them. If the lord of the realm has other land under his control, he may very well have decided that the trading port really is just that - a port. Having built only that which was necessary to service the port and the needs of the sovereign Whoever, other resources could have been used to improve other parts of the land.

Note that the 5000 number doesn't actually require that the place be awful - just that it's not sensational, and that it's not old. If people in the area are pretty sure that the continent they live on is the coolest thing ever, the world be damned. If it's been there quite a while, obviously, the population stands to have grown - peasants didn't just up and go somewhere else without good cause.

Please note - I am by no means suggesting that anyone here is wrong. But the staffing requirements of the port aren't as high as all that, and the rest depends on history and attitude.

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Here is a nifty calculator for medieval demographics. It is not FOR harbor cities but it gives you a start point on which you can extrapolate :).

https://www.rpglibrary.org/utils/meddemog/

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Ancient Alexandria might not necessarily be medieval, but it was around 1,000ha for a population of 300,000 free persons during Antony's time.

1,000ha is fairly large, about 4 square miles and functions geographically & historically similarly to yours.

Alexandria's port was about half that in additional area.

Port design - keep in mind that your geologically walled settlement would have to have access to the port, and that since you are on the edge of the sea, you should have a protected, multi-use port, for seafaring and river transport. Consider bringing the port into your island somehow? Such as a gap in the wall, or more fantastically a very large tunnel that opens up inside your island, so anybody coming and going must go through that tunnel.

Growth - Don't forget to leave space for further growth, because you'll start getting shanty-towns growing up along the interior walls. Well, who's to say that's a bad thing.

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  • $\begingroup$ Re the walls, I'm picturing a gap in the crater allowing ships access, with 1/4 to 1/2 of the crater filled with water, and the rest being the land the town/city is built on. Gates cut in the walls next to the town allow access to the bridges to the mainland. $\endgroup$ – Adeptus Apr 21 '15 at 2:23
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    $\begingroup$ You may even consider something to allow / block ships. A massive gate might be very cumbersome, but a giant chain was used to block the Golden Horn blocking ships from passing into the Bosporus strait from the Black Sea. $\endgroup$ – Mikey Apr 21 '15 at 3:00
  • $\begingroup$ @Adeptus from your description, I would recommend a good radius of 3km, to allow for growth, or (if you want support during siege, like some outer farms, etc.), 4km radius - inside the walls, and inclusive of the port. This is well within the size of a small impact crater, but beware: for the waters not to have washed it away, it needs to be fairly recent and in hard stone! $\endgroup$ – Mikey Apr 21 '15 at 3:04
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You can use something like Venice and Melaka as a baseline. Population and size of a trade hub will probably be based more on

  • Traffic, e.g. Malaccan and Singapore were situated between the best India and China trade routes
  • Resources worth trading. If the only thing the land produces is wheat, trade would be really weak. Spices, exotic food, precious metal will really improve trade.
  • Living quality. The more advanced civilizations will have plenty of wealth and merchants. Poorer ones will be self sustainable.
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