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So I was playing Deus Ex and came across Hengsha, a city so large they built another city on top of it. This got my mind wondering about the practical uses of such a city.

Let's assume that the government has the political power to build this city, and the resources and technology to do so.

Why would this expensive project be undertaken instead of just building another city further away and building a transit line? Or doing terraforming by altering the landscape to give more room/building on water?

In addition, besides the standard rich people on top and poor people on the bottom, what social issues/conflicts would such a city provide?

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  • $\begingroup$ An underground city beneath another city? is not a duplicate because this question is about building on top of an existing city, not burrowing underneath. $\endgroup$ – QuyNguyen2013 Aug 4 at 22:19
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    $\begingroup$ Asking Why is too opinion based to be answered properly. As the creator of your world, you need to decide WHY. $\endgroup$ – Shadowzee Aug 4 at 23:16
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    $\begingroup$ That is a lot of separate question, $\endgroup$ – John Aug 5 at 0:00
  • $\begingroup$ You really need to define what you mean by "on top of". Many older cities (or abandoned ones: see "tell") are built on top of layer upon layer of older cities. And in the modern era, what else is a skyscraper? Especially if you add a few skywalks between buildings... $\endgroup$ – jamesqf Aug 5 at 17:15
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I imagine it less as a purposeful city-planning choice and more as a gradual transformation.

First, as the city became prosperous, wealthy elites bought up apartments near the city center. In their culture, even more so than in ours, the height of a person's apartment above the ground was a significant status symbol. It became common for the bottom floors of buildings to be much poorer than the top few floors - the height mattered more to the (literal) upper class than the building's location.

As technology improved, buildings grew taller and more structurally sound. Long elevator rides and the unpleasantness of mingling with (gasp) the common folk prompted the rich to prefer buildings with skyways, bridges high above the streets that connected the upper floors of skyscrapers. Extravagance and architectural possibility grew in tandem, and before long, it was common to look up from the street level and see the bottoms of little courtyards where a few of the skyways intersected. Small parks and sitting areas for the wealthy to pause at on their strolls between the buildings, to chat with their well-to-do peers.

But why stop there? The rich wanted every luxury imaginable without having to travel to the surface to get it. They wanted markets and cafes up there, theaters and stadiums. And what business wouldn't leap at the chance to set up shop right outside the windows of the city's deepest pockets?

The walkways and open areas grew and spread until they blotted out the sky for the unfortunate wretches down below. And as a not-entirely-unintended side effect, now the inhabitants of the upper city don't have to look at them. And good thing, too, you might hear an older resident say. I remember when you could look over a banister and see trash piled up by the streets thirty stories down - why, it was near enough to make a man cough up his caviar!

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Proximity.

Cities spread out laterally all the time. People want spaces to work and live in proximity to the original city and so the area occupied by the city grows.

If the same forces were at work in your city but horizontal growth was not possible, the city would grow vertically.

I can think of a variety of reasons why horizontal growth would not be possible.

1: No land. Your city is in deep water, and the supports holding it off the ocean floor far below cannot be duplicated. New city needs to stay on the existing supports.

2: Defensive fortifications. Building outside the city wall is dangerous. The wall cannot be extended. Only vertical additions can stay within the wall.

3: Something else is outside the city which precludes lateral spread. Maybe the land is owned by others who do not want urban sprawl and have the power to prevent it. Or it is unbuildable for some other reason - quicksand, monsters, mud volcanoes etc.

4: City is spinning. Your city is turning on a vertical axis. Extending out horizontally means new additions will have too much angular momentum and they will fly off. Non-spinning city additions are worthless, of course. You need to stay on the vertical axis.

5: City is floating in the air. I picture a Bespin-like cloud city. Adding constructions underneath the city that is already there poses less risk of screwing up the center of gravity.

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I'll answer the last question. The bottom city definitely has to be the slums, or something close to a neglected area. There will be very limited, if not an absence, of sunlight that reaches down. This means that maybe the top city can have control of what times the bottom city gets light, essentially controlling the lives of those on the bottom. Consequently, only the top city can grow food and anything else that's light dependent.

Structural degradation of the top city will always have the possibility of falling into the bottom one.

Very intense winds will ravage the bottom city. Not too sure of the science, but perhaps this results in frequent tornadoes or dust storms.

Garbage from the top will definitely end up on the bottom, no point in adding more weight stress on the supports.

I'd say the top city will play more than just the rich and snobby, they'll be considered as literal gods to the people on the bottom. They are the bottom's only supply of food (any supplies transported down to the bottom will literally look as if the supplies are from heaven), they control the bottom's day-night cycles, and after years of miseducation of the bottom, they can be tricked into thinking all those tornados are god's wrath.

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  • $\begingroup$ Could you expand on why the winds would be more intense for the bottom city? Intuition would suggest the reverse. $\endgroup$ – Whelkaholism Aug 5 at 15:03
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A similar (though not exactly identical) thing actually happened between the 13th and 15th centuries in Edinburgh, Scotland. The desire to be close to the protection of the castle and inside the city wall led to the construction of tall (7 + story) houses, some of which having multiple entrances and exists at different floors.

Surprisingly, the classic fantasy trope of 'lower classes on the bottom, rich on top' was only partially true - the very bottom floor (being next to the open-sewer streets) was reserved for the dirt poor and the animals, while the floor above that was definitely lower class. However, the 3rd or so floors were often lived in by the wealthiest inhabitants of the city, while the floors above consisted of a rabble of middle class and poor as well. A few reasons for this were

  • The tops of these buildings were often built shoddily and out of wood, so very prone to catching fire, while the lower floors were stone
  • It's exhausting to walk up 9 flights of stairs every day
  • In the (likely) event of a fire, you can get out if you live at the bottom
  • Buildings were massively interconnected, so it wasn't like you had the upper floors shuffling past you every day.

Often buildings would literally be cut off halfway up then built on top of, such as the Edinburgh city chambers. This was the easiest way available to build a large government building in such a crowded, archaic town center.

Edinburgh city chambers.

In short, the thing driving the vertical construction was lack of space, and the need to accommodate archaic old-style unplanned city streets.

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Building vertically

Stacking cities could be viable for a number of reasons, they generally revolve around the cost of building out due to the scarcity of usable land outweighing the cost of building up:

Say if your planet was covered in swamp, fortifying the ground could be a monumental task that required:

  • Draining of large swathes of land.
  • Reinforcing with expensive materials like rock.
  • Planting of biological root systems that would hold the

The majority of the planet was irradiated/toxic. New land would require:

  • Purging of radioactive/toxic chemicals.
  • Removing the producers if toxic chemicals are a product of a biological process.

The planet had life that was incredibly dangerous to the cities survival and whose defense cost increases faster with increase in circumference than building cost increases with height.

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I doubt if anyone will ever build a city on top of another city, but it seems to me rather reasonable to save space by build a city that is a single giant building with many identical levels stacked one above the other for structural support.

A city in a giant building that is one hundred stories tall and occupies a ground space one mile square will have a total of 100 square miles of floor space, equal to a single story building ten miles by ten miles.

A city in a giant building that is one hundred stories tall and occupies a ground space ten miles by ten miles square (100 square miles) will have a total of ten thousand square miles of floor space, equal to a single story building one hundred miles by one hundred miles.

A city in a giant building that is one thousand stories tall and occupies a ground space one hundred miles by one hundred miles square (10,000 square miles) will have a total of ten million square miles of floor space, equal to a single story building 3,162.27 miles by 3,162.27 miles.

In short, by building a city in a giant building with many identical levels stacked one above the other for structural support, a city large enough to hold the population of an entire country, continent, or planet could be built on only a tiny fraction of the total land space, freeing the rest of the land space for other purposes such as farming, maintaining natural ecosystems, etc.

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