What problems would a truly gigantic fantasy city encounter?

Question

Supposing a world that falls somewhere between low and high magic, what would stop a city of truly epic size from existing?

• The city is a city-state that has existed for millennia.
• It is pretty much the mercantile centre of an entire continent, connected to the capitals of other empires via magic portals that are big enough for caravans to pass through.
• Naturally it is also a port city.
• It is surrounded by thick walls, but the area enclosed is several hundred square miles
• Although magic exists, and is used by mages, they aren't as frequent as in let's say Faerun. It however used to be more powerful in the past, so the original founders could build huge structures, like city walls that are a hundred yards thick and forty yards tall.
• There are many cultures living in the city, not all of them friendly to each other. Some live in enclaves, others mingle.
• The tech level is late-medieval, early renaissance, give or take. Gunpowder exists but is still a closely guarded secret.

What practical problems would such a setup encounter? Beyond the fact that it can't realistically stretch beyond its existing walls, what are the main limiting factors of growth?

Update:

A bit more detail:

• Portals always take you to faraway lands. New portals are prohibitively expensive to open with the present-day, weaker magic but existing ones can be "barred" to prevent invasions and suchlike.
• Let's say the population would be between 5-10 million
• Portals could in theory transport anything that fits on a large cart but you need to get the stuff to the portal first, so transporting sewage across them doesn't sound feasible.

And to narrow the question down to a finite list of problems:

• How would it be possible to organise food supply on this scale?
• How would the city raise funds? Would taxing citizens even work in such a dense urban environment?
• What would be the other high volume resources a city needs to survive beyond food and water?
• How would people move in a city that big? Would they by and large keep to their own neighbourhood, thinking about one day visiting the other side that is a day's walk away?

Answer 1

Not Enough Roads

The city needs stuff (like food) brought in and stuff (like excrement) brought out. They build roads connecting the city to the not-city.

The line-square law says that a really big city cannot possibly have enough roads coming in and out. No way in heck. Absolutely not. No chance. Not even if God built the roads himself.

When the city was one mile wide and one square mile in area, a single road was plenty. When the city grew to two miles wide, it was four square miles in area. The single road was not enough so they built three more. Four roads for four square miles of city. Things worked fine for a few years, until the city grew to be three miles wide. Now they needed nine roads.

Big cities need more roads. So what's the problem? The problem is that -- proportionately speaking -- bigger cities have less space to fit all those roads. Once the city is $n$ miles wide, it needs $n^2$ roads. But for a square city say, there are only $4n$ miles of perimeter to fit the roads.

Imagine the perimeter of the city (shown in green in the image). Each road cuts through some section of that perimeter (red). This limits the number of roads you can have coming in and out of the city. And since $4n$ grows slower than $n^2$ you eventually hit a critical size where the perimeter is too short to fit enough roads. The city starves or chokes on its own excrement.

This is the same principle behind why a mouse needs a fur coat to stay warm, but an elephant is perfectly happy to be bald. The elephant has small surface area relative to its volume. There is less area for heat to escape through.

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The Mathsy Bit

Suppose a road is width $w$ and each square mile of city requires one road. So a city of width $n$ needs $n^2$ roads which together take up $w n^2$ of the perimeter. For a square city the perimeter is $4n$ so once $wn^2 > 4n$ the city cannot have enough roads. This simplifies to $n > 4/w$. This is the upper limit on the size of a square city.

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Edit: The square-line law can be overcome by making the city 3D. For example put the farms or cesspits on the surface of a sphere with the city on the central disc. Both the city size and farm size scales to the square as we increase the radius. Diagram:

City in blue. Farms in orange. Connect the roads to the farms in a grid over the city.

Answer 2

Shit.

That is all.

Cities produce a lot of it. Especially mercantile cities. You've got not only the shit of myriads of individuals, but also turds of local domestic animals and a wide variety of livestock dung. You've got to do something with it which means some kind of crapophoric infrastructure.

Your city is a port, so dumping all that shit into the harbour or the estuary would not be a very amusing solution. The Great Stink of London (1858) resulted from the very same problem. There are loads of other waste products too, from toxic chemicals to bits of dead animals to ordinary waste.

A sewer won't help much either because your city is close to the water's edge which is the natural terminus for such a system. Cesspits are no better, as all that crap just accumulates.

I would suggest a system where nightsoilmen, cesspit diggers and muck rakers of various sorts collect up all the waste they can and bring it to a kind of under ground rail road which would transport the sludgy ooze in wagons to places outside the city where it can be processed. The wagons can be drawn by beasts of burden or slaves, depending on your city's economics.

Once outside the city, all that shit can be allowed to dry. The resultant liquid effluvia can be dumped back into the river whilst the solid can be sold off to farmers as manure for their fields, or even sold as fuel. There are also gaseous products, such as methane, that can be collected and used as fuel. This could be transported in hermetically sealed sacks.

Answer 3

I'm focussing on things that specifically relate to supporting large population densities.

City issues

Transportation

If you have people, they will want to go somewhere, which means roads, which means you'll have maintenance crews. Your big walls may be pretty, but walls block things, while roads connect them.

What does your public transportation look like? How does the typical person go to work? Cross the city? Leave and enter the city? Do you think that enough food gets into the city on wooden carts? If you're used to American cities, look into other countries for realistic models of public transportation. Please get more creative than using brontosauruses as busses.

Look into a magical adaptation to rail systems. Maybe you can use waterways or levitation.

Water and food

This is an essential need for living. Water delivery isn't something you can skimp on, and it becomes an exponentially larger issue when you have large populations. You can only pump so much water through a pipe. If you make bigger pipes, you need to dig deeper to accommodate that pipe. You also need to figure out how to get food distributed to so many people. This is why we now have supermarkets.

Verticality

At a certain point, your population will want to go upward to fit more space in the small area. Hong Kong has minimum building height of 60 stories on the inland. This means you have to figure out how things get moved up and down, which means good elevator "technology" and identifying how you're going to build upwards without immense steel foundries. Try walking a bucket of water up 40 flights of stairs to get a feel for this issue.

Sanitation

You need a solid set of sewers to take that much sewage away from where it's being created, and you need a place for that sewage to go. If you're on a river, then there is a temptation to put your sewage there, but that results in your river being nick-named 'the big stinky' or something similar. If you have sewage being generated in great amounts, it actually benefits you to have sewage treatment plants, not just to manage your diseases, but also because you can produce some fantastic fertilizer by separating out the organics from the water.

You also have to figure out what happens to a society's garbage. If the city doesn't figure it out, you WILL wind up with vacant lots becoming trash dumps. Socialist model has trash services that set standards. Capitalist model charges people for that service, and they can vary the standards by paying more. Both of them need police to enforce the rules.

Disease and vermin

Disease is kinda like criminals, except the criminals are tiny. You need a special police force to figure out where the diseases live and which people are harboring and feeding the criminals.

Vermin are a side effect of poor trash disposal practices. They congregate in areas where people shed their leftovers, because leftovers are yummy. This might be trash dumps, sewers, or just poorly stored food. You need people who know how to spot the problems and wipe out the breeding grounds, otherwise the vermin will inevitably start feeding on people and spreading disease.

Crime

On this one, I'm going to recommend some reading:

• Pratchett's "Guards, Guards!"
• Glen Cook's "Garrett, PI" series
• Judge Dredd They're all well worth the entry fee. The first involves building a police force in a fantasy world, the second describes the kinds of crimes that you can expect to happen, and the last specifically deals with overpopulation.

There are other challenges particular to dealing with crimes in a large city because some areas (burrows, vertical living complexes, gated communities) pose challenges and threats that you don't get in smaller populations.

Postal services and other communication

You don't get big cities without a decent communication network. Maybe this means a postal service with drop-tubes, homunculi sorting letters, or airways filled with owls. Remember that your characters aren't the only ones who have to send letters.

Rioting

When you get enough people together, they will inevitably find a reason to get angry and destroy things. Keeping the reasons to a minimum helps, but more often than not, the people at the top just don't notice how annoyed the poor people are unless the poor people wreck some shit. Riot control involves having a militia to minimize the damage. For authoritarian regimes, it involves having riot police whose job it is to teach the miscreants a lesson, but fiction is filled with clever methods of "harmless" riot suppression, like riot foam or something that turns all of the surfaces slippery.

Terrorism

Terrorists come in two categories. The first is a form of psychological warfare operations, and the other is advanced rioting. In the first case it's just an advanced policing issue. The population recognizes the harm, and will work with the police to prevent it. In the second, the people ARE the insurgents and the population is sympathetic, which results in trying to keep the entire population in jail at once. It is often a major plot point finding out that the former is actually the latter.

Social/political issues

There are a lot of things that any large group of people will need to figure out. These are not inherently city issues, though, so I'm not going to go into much detail.

Poverty

Poor districts need more police, they have a higher population density, and they have greater need for social services, but this happens for a settlement of any size.

Taxation

How is money collected to run all of this? Do you even collect taxes, or just enslave random people to do the work?

Answer 4

What can magic do for us?

In the spirit of the famous question "What have the Romans ever done for us?" we need to see what magic can do to alleviate the problems of an early Renaissance mega-city state. Put another way, how can "medium-low" magic solve the problems that were solved by industrialisation.

Magic Portals: Having magic portals that allow caravans to pass from the capitals of other empires to this city are useless from the point of view of big-city logistics. All of these big cities have the same problems - a vast appetite for food and clean water and a need to dispose of sewerage and rubbish. Connecting the cities to each other with high capacity portals does not allow any of these problems to be solved - none of them have food or water to spare for the others and they definitely do not want to receive more rubbish or sewerage. What would make more sense would be to have the high capacity portals lead to crossroads in the middle of farming areas a hundred miles away, while small, low-capacity portals connect to other cities to cater for the much lower volume of trade in luxury goods.

Sea/river travel: Making the mega-city a port city is vital. The next step is to use the magic to augment the supply capabilities of the merchant fleet. Use magic to control the winds that each ship is sailing on, have magically powered cranes on the docks and most importantly have weather divination and communications to allow efficient and safe routing of vessels. As mentioned in other answers, waste disposal is a problem, so use the merchant fleet to carry away all the various types of waste on their outgoing trips, then use magic to sterilise everything before they bring food back to the city.

Land travel: A mega-city cannot afford to use normal beasts of burden for internal transport - their requirement for food and their output of biological waste are unaffordable in a city where both issues are already critical. (Normally the fecal matter of horses etc is welcome as a fertilizer, but in a high-density city there is no room for vegetable gardens to take advantage of it.) Magic needs to provide an answer, with magical constructs acting as beasts of burden (assuming that you do not wish to go all the way to magical self-powered wagons, otherwise known as "cars and trucks"). These could be anything from undead horses to ox-shaped golems depending on what "flavour" of magic society is desired, the important thing is that they are not consuming anything or producing waste. The same considerations apply for caravans entering the city, at least within a few days of travel - the volume of traffic will not allow for biological beasts of burden to be fed in this area, as the land is too valuable for intensive farming purposes. These animals also need to be fast - as per Daron's answer, there are not enough roads to allow supply along them if traffic is moving at walking pace. So, elevated or underground high-speed roadways for travel between districts before automaton-powered carts peel off into the local streets are required.

Heating and cooking: One aspect that I have not seen mentioned in other answers is the demand for firewood or other combustible materials such as dried animal dung for cooking and heating. Even without worrying about heating, the mass of firewood required for cooking will typically exceed the mass of the food being cooked. This is why all pre-industrial villages, towns and cities maintained forested areas very close to the residential areas, typically much closer than the fields where grain was grown since the wood is much heavier to move than the grain. Which means that not only do you need to supply wood for 5-10 million people to do their cooking and heating, but the population will be choking on the fumes from that many fires in such a relatively small area. Magic must come to the rescue here, with some cheap, clean option to provide heat for all of the citizens, or the famed London smog will seem like a breath of fresh air to the citizens trapped within the smoke cloud held in place by forty-foot high walls around the city.

Governance: This seems like a non-issue to me, regardless of magic. The rulers at the top need to delegate duties over a manageable area to people underneath them. The Roman Empire and other pre-industrial empires managed it over vast areas with no magic at all, it will be much simpler over a much smaller area. As for taxation - income tax is a very recent invention. Historically, tax was primarily on imports or land, both of which the rulers of the city will need to keep a very close eye on and can tax appropriately.

Answer 5

Take your pick

• Disease / Sanitation
• Crime (organized or otherwise)
• Other Cities / War. You have walls, but you also have portals that let people through
• Poverty / Hunger (There might be enough food in the city, but it doesn't mean everyone has some.)
• Terrorist groups
• Over population (If it's really great, people will live longer, and it will get overcrowded)
• Exploitation of: children, minorities, poor people, etc.
• Water scarcity, unless you can direct rivers through portals
• Financial problems (depending on how your world is set up)
• Government / Politics

Depending on the details of your world there could be a lot more.

Answer 6

Fire is a potentially unmanageable risk in this scenario.

The buildings cannot all be made of stone. Between the scale of the city and the number of fireplaces, candles and forges it seems to me that sections of it would surely be burnt to the ground quite regularly.

Perhaps fire breaks and boulevards can help, maybe there are fire safety laws that help manage the risk, but a city on this scale at this time period feels like it must be running serious fire risk.

In historical times, the most risky industries were grouped, frequently outside city walls, but a city on this scale that may be unmanageable.

Answer 7

It's like Seoul, which is decreasing in population

Look at modern first world cities with declining population, for example: Seoul, with a birth rate of 0.68, it's one of the fastest declining population cities in the world. There's a huge concentration of population in Seoul, compared to the rest of the country:

However, the birth rate of in Seoul, at 0.62, is the lowest in the entire country. Just to maintain a steady population, Seoul would need a birth rate of 2.1, and so the population has declined over the years:

Every other province in Korea has a higher birth rate than Seoul,

The real question is why? Seoul has very effective transportation systems, sewage management, low crime, long life-expectancy, and a very highly educated population, the internet is fast, most people have smart phones, what could go wrong?

There's a few reasons for declining population, most severely in Seoul:

• The rent is higher in Seoul than other parts of the country, when faced with a choice of where to live for rent, a simple economic decision often drives from housing cost.
• The cost of education is higher in Seoul, most Seoulite parents send their children to several after-schools for English, Math, Science, a sport, and to learn an instrument. There is intense pressure for Koreans to "fit-in" and so it is normal for children to go to many after schools.
• Long working hours: Korea has some of the highest numer of working hours in the world. Parents don't have time to raise children.
• Strict immigraiton laws: Korea, like Japan, has remained a mono-culture for hundreds of years, as of today 96% of the population being of the same culture. It's very difficult to qualify for a visa to live as a foreign resident in South Korea, and they usually require things like having a 4 year bachelors degree, having a certain income level, no crime record, and they require to have a work-contract and require you to leave the country if you quit that job. I know you said your city was more multi-cultural, but perhaps the country it's in is less inviting.

In conclusion, perhaps your city could decrease in population for a reason similar to some modern day cities that are also declining in population.

Answer 8

Total National Population

In the preindustrial world, the inefficiency of farming meant that it generally took about 10-20 rural people to support 1 urban person. Furthermore, Rural places need local urban centers to function as trade centers and sources of common goods meaning that you rarely see more than 10-20% of a large nation's urban population crammed into a single city. The bigger your nation, the smaller % of the population you can expect to be in the capitol for logistial reasons.

So, lets say you ruled the ENTIRE WORLD in the late medieval period. No, not just Medieval Europe, I mean everyone: China, the Aztecs, the Incans, the Mali Empire, etc are all serving this one city, you would have access to a total population of ~300-400 million people through all these portals. Since most or all of the magic is in your city, we can assume that farming is still done the normal way such that you can only support an urban population of 15-40 million people. With a large number of portals, we can assume that your capitol can act as a "local" urban center for many rural people who live close enough to a portal, but unless you have thousands portals replacing every single town and city on the map, most rural communities will still need local towns of urbanites to act as trade, production, and administration centers, and those towns folks will consume most of the excess agricultural produce. This means that only about 1.5-8 million people could actually live in your capitol.

However, you've restricted it to only a single continent, and not a world network, meaning your actual urban population for the whole empire is probably closer to you can only support an urban population of 3.75-10 million people giving your capital a maximum population of 300,000-2,000,000 people under normal circumstances.

Since this falls short of your minimum goal of 5 million, it means that you need to offset some of the need for local urban centers with portals strategically situated around the continent where towns and cities should be. The average urban center in Medieval Europe was about 5000 people, that means that you'd have to have about 600-1000 portals taking the place of towns and cities across your empire to reach the minimum goal of 5 million people. Rough estimates say that this is somewhere between impossible and 50% of your total urban population living in one place. Even the best case scenario here is very unlikely IMO since most urban people will probably prefer to settle around the portals instead of inside the capitol where all the over crowding is... but something in the 2-3 million population range may be believable.

Answer 9

Too Big For Network Effects

Although cities are very useful for packing lots of useful things in a small space, there's a limit in that these things will occupy a bare minimum of space. Have enough of them, and they will take up a lot of space. Of what advantage to you is the endless spring of pure water if it takes a week to get there and a week to get back to your home?

Building up and with bridges between buildings may increase its size usefully, but eventually the access is technical, and therefore there's no real benefit to the city's size past it.

Answer 10

In addition to other problems mentioned in other answers:

the city will be almost ungovernable

(which might make it very interesting place!)

I am sure such an enormous city was not built in one go with just a single loooooong wall around the huge area. It started smaller, and it probably had a wall surrounding the smaller area. Then it grew, had more and more settlements and businesses outside the wall, which needed protection, so a new wall was built. There were probably other towns, too, because the area is huge, and some had their own walls, and they were engulfed as the city grew. Much like London has grown, although it has not needed so much walls.

This leads to a governability problem: walls and engulfed towns form areas and niches that provide opportunities for people who want their own little kingdoms inside the city. It will be like a large kingdom wirh rivalries compressed into a city.

Which can provide endless opportunities for stories.

Answer 11

Your city doesn't sound a lot different than many cities that exist. As always, if you want a large city, magic it. Don't explain how it works. Just show that it does and move on.

For example, someone mentioned sewage. And you mentioned magical portals for transportation. This problem solves itself. And why wouldn't this be one of the first problems the mages would solve? The 2nd problem probably involves getting food and clean water into the city. More portals. This is exactly what they would have been used for.

Now the road situation someone mentioned is largely alleviated.

Like many large cities, your city could be an aggregation of smaller, generally stand alone boroughs. Each would have its own culture and customs. Think Queens, Beverly Hills, and Compton. This gives you plenty of different locations for stories.

The question I have for you is why does this city exist in the first place. It isn't easy to put that many people in a small-ish space. Why would they do it? The answer is again, the portals. They are ancient. No one knows who created them or how they work. They just know A connects to B, and possessing at least one end of a portal provides remarkable capabilities. For example, you could create a desert paradise if you routed a river into a portal.