I'm aware that variants of this question have been discussed before (such as "how to build a self-sustaining medieval city?"), but my question comes at the problem from a slightly different angle.

Imagine that you are tasked with taking around 10,000 people, and imprisoning them within an enclosed city-space, from which they cannot get out, and nothing can come in (how that is to be accomplished is not relevant to the question). In other words, the isolation of the city is absolute, and after you've done your job once, even you can't intervene any further.

The only condition is that the people or their descendants should not die out - i.e., the enclosed city must be able to sustain itself at a basic level, and for a couple of thousand years. You belong to a rather high-technology civilisation set in the distant future, and so you have a near-God-like freedom in designing the elements of the city - the location (for weather and climate), the soil, availability of water etc. You cannot build anything specific for these people - all of that they are meant to do for themselves.

The people in question are, at present, at the same technological level as you, but there is an expectation that forced isolation will, over generations, lead to a sharp regression.

I've studied the previous discussions on similar topics, and I have a pretty good idea of the ratio of farmland to population (and other similar considerations) when it comes to the question of food. What I'm specifically concerned about is (a) designing a sewage system, (b) sources of clothing, and (c) the disposal of dead bodies. Any suggestions on those counts - and of course, any element I'm missing out in ensuring that the city survives - would be most welcome.

Edit: A few clarifications in response to the questions below. By "enclosed", I am only referring to a physical enclosure - say, by an impenetrable wall, or, as someone suggested below, a force-field. Sunlight etc, in the normal way, is all available.

One exception to the "enclosed" rule is the existence of a river, that runs through the city, and passes underground (and ultimately beyond the barrier between the city and the rest of the world). The source of the river is not known (to the inhabitants), and the dominant explanation for its existence is (obviously) a religious one.

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    $\begingroup$ You may wish to clarify the absolute-nothing-in/nothing-out rule. Water, food, trash, sewage, air -- all must, to some extent, pass in or out. Example: A meter-diameter pipe for water or sewage is unlikely to last thousands of years, and is also large enough to pass people. No-trash-out means the town may be buried in their own trash (and corpses) within a few generations. $\endgroup$ – user535733 Dec 3 '18 at 0:22
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    $\begingroup$ @user535733 You're thinking more of modern cities. Medieval cities would recycle waste. Corpses break down. Sewerage would be retained as nutrients for crops. Medieval cities were very stinky places. This one would set a new high in stench. $\endgroup$ – a4android Dec 3 '18 at 0:29
  • $\begingroup$ @a4android agreed. Precisely why I brought it up for clarification. $\endgroup$ – user535733 Dec 3 '18 at 0:52
  • $\begingroup$ Welcome to Worldbuilding, GautamBhatia, it's excellent you have thought the issues for a self-sustaining city. Since this city exists in the future of several millennia hence. One question: are tools, plans, & materials available to city-dwellers so they build systems, facilities, perhaps machines for themselves? $\endgroup$ – a4android Dec 3 '18 at 1:02
  • $\begingroup$ @user535733 I realize rereading the question this city is in an advanced future world. While I assumed this was a medieval city & even a basic future prison-city will share many characteristics of a medieval city it isn't the same thing. Sorry! My fault! I should read the question with more care. $\endgroup$ – a4android Dec 3 '18 at 1:05

Recycling is the easy part, as this is a future society, we can rely on atomic manufacturing techniques which all the full disassembly and reassembly of atoms. This nicely avoids the recycling issue, at the expense of power. Power can be worked out by having some sort of near-infinite crystal-void-energy-make-it-up power supply. Think a ZPM in Stargate.

The length that this power supply operates on, its peak power, and reliability is the determining factor in how long the city could survive for (technologically speaking), as once sealed the city likely does not have the ability to make more.

Cities do not exist in isolation. A city develops to supply a point of exchange, usually trade, or labour, to the region it exists in. When either or both of those run dry, the city dies. This scales up and down from village/township right through to mega-city.

To enclose a city so that even information cannot enter/exit essentially stops trade. It also ham-strings labour. So there really needs to be an artificial supplier of labour demand. The most likely such supplier would be a religious organsiation. We have many examples in our history of mostly self-sufficient edifices such as conclaves, monasteries, etc... Each was a product of religion.

The religion would probably operate with a caste system, or pseudo caste system. Free thinkers will be promoted to a prison (or gilded cage) neutralising their threat. All of the required professions will be trained by the religion, and workers will be given few freedoms and encouraged/demanded to participate in numerous brainwashing/community activities. Those who conform will be public praised, and those who don't will be made into public spectacles.

As the religion is primarily focused on maintaining control, and benefits from a relatively naive community, it will naturally dismiss and reduce the technologies in general use. It will maintain many of the technologies for itself and use these to re-enforce their own status. Some of these technologies will stop being understood even by the elite in the religion.

Human Resourcing is the tricky part. The population will naturally have years with more births, more deaths, more disabilities (permanent and temporary), etc... That means some decades there will be less workers than demand, and other decades will have more workers than demand. With ten thousand individuals in your society, that roughly means a work-force of 7000 people (3000 people are babies/children/elderly/sick). Of those 7000 all positions in growing/manufacture/transport/management/etc have to be filled at least twice, in order to have some elasticity built in for years low in available labour.

For years with excess labour, the religion will need to perform some form of public/spiritual works. These would be golden years for the religion displaying true faith, etc... The problem is that each spiritual/public work will consume resources (limited in supply) and space (also limited in supply). Destroying, or allowing such works to deteriorate will cause general resentment in the population.

  • $\begingroup$ Thanks very much. You have it spot on that religion ends up playing a major role in this city/society. However, in such an enclosed city, wouldn't a lot of people end up labouring on farmland or be involved in basic manufacturing activities to fulfil clothing/living needs? And wouldn't that labour need to be directed by a secular and somewhat centralised authority? $\endgroup$ – Gautam Bhatia Dec 3 '18 at 14:32
  • $\begingroup$ Unfortunately no. You have a futuristic society with a wealth of technologies. Many of those could be used to reduce the burden on farming. Even using labour intensive technology, most of the inefficiencies in farming are related to climate control, water/fertiliser, and food storage. So with only a few key technologies (such as refrigeration) acquiring food will not be much of an issue. It will require knowledge of manufacture/maintenance of those technologies, but that really would only require a few people and books. $\endgroup$ – Kain0_0 Dec 3 '18 at 23:03
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    $\begingroup$ Secular Society is actually a new concept for dealing with multiple distinct cultures, both within your borders and abroad. Ten thousand people trapped for even a few generations will rapidly blur cultural distinctions and form a single culture. This might be done initially through inter-cultural relations (multi-culturalism), or through genocide. Any stratification of the people in such a small population will lead to heavy inbreeding, endangering the cities future. As most people are applied to survival, that leaves few for governance and religion, often fusing them, or forcing close ties. $\endgroup$ – Kain0_0 Dec 3 '18 at 23:14
  • $\begingroup$ The authority will be somewhat centralised, but not because its an isolated city. It is because it will roughly need 20-100 managers. These might be called councilors, priests, aldermen, etc... Roughly speaking a village needs 1 full time leader for every 100-500 people based on the technologies available, and the kind of social structures implemented. The people though will still hold a lot of sway as each of those leaders has family and extended family which could easily account for 100+ individuals. $\endgroup$ – Kain0_0 Dec 3 '18 at 23:25

Not a city. A town with farming villages.

With 10,000 people, you can forget internal combustion engines and most other heavy industry. That means most people will work in agriculture, toiling in the fields from dawn till dusk.

  • For sustainable agriculture, you need quite a lot of area. Enough to have not just crop rotation on your fields but also rotating areas between farming, grazing, and forestry on a centuries-long schedule. That handles your burial and sewage problems, too. Human "nightsoil" on fields can cause medical problems, so put it into forest reserves.
  • Clothing is wool, cotton, or the other traditional methods. If there are animals there will be leather as well.
  • There might be some iron and even steel production and blacksmithing. At that scale, A hill with iron ores would be plenty for a couple of millenia.

A Long-Term Balance is Difficult.

If you have enough resources for 10,000 people at a sustainable rate, then humans will breed 11,000 people, or 20,000 people. Who needs a forest preserve, cut it down and farm. What do you mean, the water table is dropping? I want my red meat on the table!

  • $\begingroup$ Thank you! Assuming that the water problem is solved by means of a river that flows out of the city below the ground, how large would you say the city needs to be to sustain the food/waste disposal needs of a population of around 20,000 people? $\endgroup$ – Gautam Bhatia Dec 3 '18 at 14:34
  • $\begingroup$ Googling got me a medieval population density of 20 people per square kilometre. That would be a circle 40 km in diameter. I guess a higher density would be workable, but not too much or people will cut into long-term habitability by over-grazing or excessive logging. $\endgroup$ – o.m. Dec 3 '18 at 18:29

Much will depend on your future society's moral & ethical foundations.

Since you are proposing a (reasonably) hermetically sealed city that ought to remain so for the next few millennia, I am going to assume that your society really doesn't give a fiddler's fart what happens to the prison-city or the people within it, so long as the population held in the zoo doesn't die out.

(If you want a truly hermetically sealed prison-city, you need to build in on or in the Moon. And with very robust city service systems.)

Keeping it on Earth, and assuming sufficiently high technology, I'd go for the simple solution. Build a 250 foot tall (and deep) force field all the way around a, say, ten square mile bit of okay land. Not prime real estate, but land that people who want to survive can survive on. Since I can't build anything for them, I'll simply create and stock a reasonable supply dump for them: basic building materials (cement, sand, bricks, stone, lumber, etc.; basic tools for working in these materials; basic reference materials for learning how to "live off the grid"). And then simply turn on the power to the force field and leave them to it! This supply dump should be enough, if they're smart, to build themselves a few great-houses, barns, sheds & workshops. Once they figure out how to make bricks or hew stone, then they can build other needful buildings.

Your specific concerns:

  • designing a sewage system: I'm not really sure why we care. Any rate, we can't design or build anything for them -- they have to do this themselves! Basically, they'll just come up with their own version of Shits Creek and eventually cover it over and make a cloaca maxima of their own. Designing a (proper) sewage system for them absent anything else proper doesn't make sense. We will have to build some kind of sewage treatment system outside the prison-city's perimeter, however. This in order to take care of any contamination that leaches through the soil or runs down the drainage from their land to ours.
  • sources of clothing: Basically the same answer: why do we care? They're on their own. They can figure out how to kill an animal and process the hide from the library we gave them; they can also learn how to husband wool bearing animals and then spin and weave their yarn into cloth. Or they can go entirely or mostly naked. Humans can survive (and can evolve) being naked to downright chilly temperatures (think Tierra del Fuego).
  • the disposal of dead bodies: Ashes to ashes, my friend. They have or can make a couple spades. All they need is two strong lads to dig a shallow trench and lay the corpse to rest in the old sod.

At the other end of the experiment I suspect that the descendants will a) be rather hardy but of a now much lower technological level and b) quite possibly still pissed off your "civilisation" imprisoned their ancestors in a big cage without considering the grave miscarriage of justice done to generations of innocent people.

Just two follow-up questions, if I may:

  1. Won't there be a problem when the initial building dump runs out?

    The initial supply dump is not designed to last very long, especially when compared to the overall length of time this "experiment" is designed to run. Since one of the experimental conditions is that we can not build anything for them and the other is that their progeny ought to survive, the least we can do is get the initial inhabitants "over the hump".

    The area selected for their city ought to contain sufficient resources (stone at the very least; sand, clay, woodlands, water as well) that they should be able to manage their own store house of supplies going forward. If some people of intelligence and foresight come into leadership roles, I don't believe the initial supply dump will be problematic.

    Any problematicity of initial supply dump size is, once the prison-city's barrier is energised, no long of our external concern. Any problems with amount or selection of supplies falls to the inhabitants to sort out on their own. They will have to make do or do without.

  2. The presence of wool-bearing animals would, I imagine, require a further extension of the city limits, in order to provide pastures for the sustenance of these animals?

    This is a good point. While I think 100 sq.mi. is a pretty good size (that's the size of Washington DC) a larger size would be possible in theory. The largest ranch in the US is something like 3000 sq.mi. Increasing the size of prison-city's perimeter that much will simply become a function of land available for the project vs private ownership of said land (if that concept exists in your culture) vs existing land occupation, use & presence of structures.

    This situation could work to advantage, however: since we are obliged to not build anything specific for these people, the previous existence of dwellings, storage facilities, barns, shops, some kind of sewage & potable water infrastructure solves some of the issues at hand.

    It will simply fall to the new inhabitants to manage the extended range of resources they've been blessed with. Armed with basic common sense and some basic resource materials (text books & technical manuals), they should do just fine.

  • $\begingroup$ Thanks, this is extremely helpful. Just two follow-up questions, if I may: (1) Won't there be a problem when the initial building dump runs out? (2) The presence of wool-bearing animals would, I imagine, require a further extension of the city limits, in order to provide pastures for the sustenance of these animals? $\endgroup$ – Gautam Bhatia Dec 3 '18 at 14:16
  • $\begingroup$ @GautamBhatia -- Follow-ups addressed in query body. $\endgroup$ – elemtilas Dec 3 '18 at 18:30

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