It's the ideal city for a siege. They have all the resources and man power required to sustain themselves during a siege.
They are able to:
Suggestions: Do they grow crops in their houses? Do they have a well? Do they have a mine?
As usual I'm willing to take suggestions.
A siege is nothing more than a test of endurance.
In order for a town to be able to do all of the things you list in a siege they would have to have access to:
Traditionally, city walls defended only the most important parts of a city, and maybe not even that, but would only allow for the population to hide away while the barbarians raid the town itself.
Some of the larger, more important capitals of Europe ended up having massive walls surround a very large area of the town, however even they could not possibly include the fields, vineyards, or even all of the citizen's housing within them.
In order to feed a decent sized city (say, 30 - 40K people) you would need hundreds of acres of fertile land, thousands of workers, thousands of farm animals, and God only knows how many tools, etc. Encircling that in a wall would be a massive undertaking, and you'd need a very large armed force to defend that wall. Who would work the fields at that point?
Not only do most cities not have access to all of the materials they need in order to achieve what you wish, they also don't have the workforce to keep all those industries staffed - a lot of those workers will have been conscripted to fight on the walls by that point.
You might be able to maintain one of those industries operating in a minimal capacity for a while if you're lucky. A mine that somehow has all its entry points within the town (not sure why you'd build a town on top of a mine, however), might be able to keep extracting some ore, or at the very least smelting and shaping the ore it has on hand for a while.
But sooner rather than later those men are going to be on half rations, not to mention busy fighting on the ramparts.
If you increase the dimensions of a city by two you increase the length of the walls by two and the area enclosed by four. If you increase the dimensions of the city by ten you increase the length of the walls by ten and the area of the city by a hundred.
So it is theoretically possible to build a city so large that it can support the men required to defend the walls forever. That can be made easier by using a shape that has the most area for the least amount of walls - a giant circular city.
Or the city might need to defend only a tiny segment of its circumference. It could be built on top of a giant mesa with vertical cliffs hundreds or thousands of feet or meters high, except for a tiny segment of artificial wall guarding the path up to the city that has to be manned by defenders.
The city would have to be vast to supply all its needs.
If you remember the legend of the Avar Rings, those vast concentric defenses included most of Hungary inside the outermost ring, and yet the Avars still raided far and wide outside their country for loot they desired, and so provoked the attacks that eventually defeated them.
Of course such an isolated city that had most of its needs locally produced and such great natural defenses and did not raid their neighbors would not be likely to be the target in a conflict.
Mathematically it's possible: Square Cube Law.
For example: Alpha City has:
You can see that the city is able to support 250 citizens where 200 of them are used like soldiers, so we only have 50 to farm enough food to feed 250, pretty hard, no? $250 \div 200 = \text{1.25 citizens per soldier}$
After some time of hard effort, the Alpha city has been able to expand their walls and now they have doubled their size!
Now the city is really happy, because they are able to support 1,000 citizens and they only need 400 soldiers to protect it (they have 600 farmers to feed 1.000 persons). $1,000 \div 400 = \text{2.5 citizens/soldier}$
As you can see the ratio of citizens per soldier is bigger than before ($2.5 > 1.25$).
Each time we double the size, the perimetre would be doubled, but the surface would be quadrupled.
Using this idea you can know that with a certain number of people it is possible to make an "isolated" city surrounded by walls: the Anime Shingeki no Kyojin: Attack On Titans is an example of that, where a huge (can seem small but, it's really big) kingdom is completely self sustainable and surrounded by walls.
I am not sure if this is relevant for your questions, but I will post it anyway.
How many soldiers should an army have to defend a city? Well, I am not sure but I don't think a city surrounded by walls would need more than 5,000 men (in medieval age only men were in the army).
Only the 7.5% of the population (in the worst cases) were soldiers. Also I am not sure if this number (7.5%) is about men or men and women. I assume the worst scenario (7.5% of men), so a city with 5.000 soldiers should have $5.000 \div 0.075 = \text{66.666 men}$.
Ugly number so I will change it to 67,000 men. If there are at least 67,000 men there would be 67,000 women, so in the city there are 134,000 persons (this is impossible for a medieval city...). Also you have to count the older people and children. Sorry, but I am not sure how to calculate that. I believe that half of the population are children and older people: 268,000 total population.
According to Medieval Demographic Made Easy a city has 62,523 persons/km2 (38,850 per square mile) so it has $268,000 \div 62,523 = \text{4.29 } km^{2}$. I think making a square city is bad, so I'll calculate with a circle.
So it has:
But we have forgotten something very important, 62,523 persons/km2 is about homes, not about the food. Luckily the same page also says that terrain is able to support 289 persons/km2 (180 per mile) AND it says "including requisite roads, villages and towns, as well as crops and pastureland" - even easier!
Extra: Also your city would have:
Now our citizens are capable of eating and sleeping, but we need to do more things.
- Food Done
- Drink, ...? Well, cities have water wells, don't they? Or at least a river or a lake... Done
- Be merry
- Mend weapons
- Make weapons
- Tend to the wounded
Now we can calculate the amount of jobs in the city with the same page referenced before.
Look at this table: (if anyone knows how to make a spoiler I would be so glad if he could edit this table)
$$ \left| \begin{array}{cc|cc} Business&SV&Business&SV\\ Shoemakers&150&Butchers&1,200\\ Furriers&250&Fishmongers&1,200\\ Maidservants&250&Beer-Sellers&1,400\\ Tailors&250&Buckle Makers&1,400\\ Barbers&350&Plasterers&1,400\\ Jewelers&400&Spice Merchants&1,400\\ Taverns/Restaurants&400&Blacksmiths&1,500\\ Old-Clothes&400&Painters&1,500\\ Pastrycooks&500&Doctors&1,700*\\ Masons&500&Roofers&1,800\\ Carpenters&550&Locksmiths&1,900\\ Weavers&600&Bathers&1,900\\ Chandlers&700&Ropemakers&1,900\\ Mercers&700&Inns&2,000\\ Coopers&700&Tanners&2,000\\ Bakers&800&Copyists&2,000\\ Watercarriers&850&Sculptors&2,000\\ Scabbardmakers&850&Rugmakers&2,000\\ Wine-Sellers&900&Harness-Makers&2,000\\ Hatmakers&950&Bleachers&2,100\\ Saddlers&1,000&Hay Merchants&2,300\\ Chicken Butchers&1,000&Cutlers&2,300\\ Pursemakers&1,100&Glovemakers&2,400\\ Woodsellers&2,400&Woodcarvers&2,400\\ Magic-Shops&2,800&Booksellers&6,300\\ Bookbinders&3,000&Illuminators&3,900\\ \end{array} \right| $$
Each type of business is given a Support Value (SV). This is the number of people it takes to support a single business of that sort. With this you can know the amount of medics, taverns, etc in your town.
In the case of this town it would be:
$$ \left| \begin{array}{cc|cc} Business&Jobs&Business&Jobs\\ Shoemakers&1,787&Butchers&223\\ Furriers&1072&Fishmongers&223\\ Maidservants&1,072&Beer-Sellers&191\\ Tailors&1,072&Buckle Makers&191\\ Barbers&766&Plasterers&191\\ Jewelers&670&Spice Merchants&191\\ Taverns/Restaurants&400&Blacksmiths&179\\ Old-Clothes&670&Painters&179\\ Pastrycooks&536&Doctors&158*\\ Masons&536&Roofers&149\\ Carpenters&487&Locksmiths&141\\ Weavers&447&Bathers&141\\ Chandlers&383&Ropemakers&141\\ Mercers&383&Inns&134\\ Coopers&383&Tanners&134\\ Bakers&335&Copyists&134\\ Watercarriers&315&Sculptors&134\\ Scabbardmakers&315&Rugmakers&134\\ Wine-Sellers&298&Harness-Makers&134\\ Hatmakers&282&Bleachers&128\\ Saddlers&268&Hay Merchants&117\\ Chicken Butchers&268&Cutlers&117\\ Pursemakers&244&Glovemakers&112\\ Woodsellers&112&Woodcarvers&112\\ Magic-Shops&96&Booksellers&43\\ Bookbinders&89&Illuminators&69\\ \end{array} \right| $$ And for the $*$ medics they would be $766$.
So:
- Tend to the wounded Ready
- Mend weapons Done (remelt broken weapons)
- Make weapons Done if they have an iron mine or copper + tin mine. They don't even need a mine at all. 5.000 soldiers won't have more than 25.000 kg of metal, right? In iron you can save it in 3.14 m3 (you can store it).
- Be merry Done?
Oh, I forgot something, the wall. A normal wall is about 2.5m x 10m. Also a good wall is made out of stone, so: (I am calculating that the wall is filled inside. It's false, so you can even use this materials to build a thicker wall.)
A self sustainable city needs all kinds of ores, metals and plant to survive alone. Luckily our city is very large, so it has a lot of terrain to find these resources, or at least to make huge stockpiles for war times.
A self-sustainable city needs all kinds of ores to be able to survive alone. With my knowledge and this question that I asked I get to the conclusion that your city needs:
I hope this could help and sorry if I went through the branches.
Not what we perceive as a city. You'd have to wall in a whole country (not impossible as the Chinese walls demonstrate, but a huge and cost intensive undertaking).
BTW, Machiavelli recommended not to build fortresses at all, but to maintain enough troops to defend the open country.
The answer really comes down to resource consumption versus resource production. All resources (whether in a city under siege or not) are somehow expressions of solar power (notwithstanding asteroids crashing into the atmosphere, etc.).
So, if your city's population is maintained within defined tolerances, and they're prepared by having sufficient garden space to grow food for that populace, and they have a waste processing solution (more preparation), and they're willing to live indefinitely without imports or exports, then yes.
I don't believe any modern technology is required, however a certain modern-like understanding of how things work would be necessary to construct a city that would withstand an indefinite siege (you'd need several specialized Davincis). And don't mistake that the city would have to be designed from the ground up (or, rather, from below ground) to withstand the siege. And the surrounding landscape, weather patterns, and a lot of other factors would have to be lined up just right to survive such an undertaking.
As someone above pointed out, your military resources would be a drain on your system, so designing the city so that its defensible by a small group will relieve that strain.
But with all those things taken into account, sure, it's really possible.
Edit: I just realized the one significant factor that one would have to take into account when deciding where to put this extended-duration siege-resistant city: salt. There's no practical way with medieval technology to capture and reprocess salt, and it's required for survival.
The only way I really see this happening, would be if you had perfect, pre-existent conditions. Such as a ring of extremely difficult to cross terrain, such as a particularly harsh mountain range, with only one or two accessible valleys which could be easily defended.
The area within the ring would not only have to be of sufficient size to support a large city, and all of its surrounding towns, villages, farmland, etc. But also large enough to support a healthy woodland, large enough to satisfy the wood, and hunting needs of local population.
But if you were to have an area like this, you could say that the soil is rich and fertile as a result of now dormant, but once active volcanoes. Such a location would also have several mountain streams, likely culminating in a lake somewhere near the centre. This would make for good farmland and therefore solve problems of food and water.
If the surrounding mountains were once volcanic in nature, it would also mean mountainside mines could well be rich in minerals commonly found only deep under ground, meaning that supplying armour and weapons for your army would not be an issue.
During peace times, an area such as this would have a healthy trading economy, due to its prevalence of rare gems and minerals. Combine that with the defensible nature of the city, and some merchants may choose to make this city their home, further helping the economy.
While this is not, strictly speaking, a city, the area would support a large city, naturally defended by surrounding geography, which would be able to sustain it's self during prolonged siege.
Yes - it's a simple answer of maths, and is similar to the square-cube rule (ie the reason you can't have 8' tall ants)
In this case it is as follows - you need X soldiers per metre of perimeter, and Y civilians to support those soldiers (call the number Z people). Each person needs A food, and that food needs B area to grow.
As the size of the besieged land increases, its surface increases at the square of the diameter, whilst the perimeter (the bit that needs defending) increases proportionally to the diameter.
So if town N is 1km across, a town of 10N will need ten times the people to defend it, but it will have 100 times the area in which to grow food and find water (and the chances of having a spring, etc in which to find water that cannot be interrupted or poisoned increases by 100 too).
So yes, a fully self-sustaining community could be besieged indefinitely it it was large enough - but you may find that by the time the numbers make sense that it's actually a small country rather than a large city by the time the maths works.
I support the "it depends" theory.
In a hurry, such as in case of a suddenly incoming siege, it's impossible to establish fully and quickly. Partial solutions might born, but as far as I can see, two resources become too rare or even non-existent at a time.
Minerals. It's simple: either you built the city upon mineable ores, and you get the necessary iron, coal, copper, gold (if it's necessary in a closed city at all, what I doubt), or you don't and as you use the processed ores and tools made out of them, sooner or later, you're out of them. Assuming you're closed, it's fatal, as fighting with wooden tools is a handicapped situation.
Plants. All types. Take a look at a medieval city. Now look at the size of fields around it. And that's just the wood. How can you implement that production ratio in a medieval city? Only with magic. Or via underground farms, if you can provide light, but that needs another resource and I'm skeptical about its feasible implementation in, let's say, 12th century's technological level. Also, if you're out of seeds, you're out of luck. And plants might be important in several areas beyond food supply: medical care, architecture-tools-etc (wood), warfare (maybe) and so on.
Walled farms might come into account but as I mentioned, they are HUGE!, and thus, walling them would insanely increase the length of border you have to defend. That'd be extremely risky.
So, under lucky circumstances, it's possible, but I'd rather keep it theoretical, unless you support it by magic or some kind of similar improvement.
For self sufficiency you need around an acre per person to grow crops on, and thats with modern farming techniques and nutritional information. Medieval times were not as productive or well informed. That kind of space is not possible inside city walls.
See: http://www.smallfootprintfamily.com/how-much-land-is-needed-to-be-self-sufficient
Medieval times weren't as well nourished either though, but at a density of 60 people per acre http://www222.pair.com/sjohn/blueroom/demog.htm Rthat leaves you pretty short. Nowadays we eat about 2000 calories a day, your beseiged population will starve on just 30, and thats if every square inch of ground space is devoted to growing food.
And that's just the food. If you want to mend weapons, tan skins, burn fuels and do other industrial stuff your space requirements grow even more.
Here's a great list to get you started.
You'll notice that blockades feature in most of them, as does smuggling in food.
To answer your question, we would have to have actual numbers rather than the generalities you've given here.
I don't believe that any siege can be held off indefinitely, and anything with walls will likely not be able to grow as much as is needed. Crops need sunlight, so growing them inside is not a viable option since glass is not common or, depending on when and where almost impossible to get. You could grow some on roof tops, but then they would be vulnerable to fire-- if I were besieging them, flaming tar would definitely be launched over the walls to cut off any food supply growing within.
Being merry and drinking involves vineyards or wheat and the space to make alcohol. That's a lot of real estate.
Making and mending weapons means a supply OR stockpile of metal. How much space that takes up will depend on how many people you need to outfit.
Tending to the wounded is more difficult than you might imagine. Healing herbs are generally found growing in the countryside and collected. Cultivation of these, which sometimes needed specific soil conditions, light and other things, was sometimes done, but you generally need a specialist to do it. You'd also need a way to make cloth to bind wounds. That means a supply of sheep, which you would have to feed.
The circumference of the city will have to be manned, even if the walls are stout, and absolutely you will need, as others have pointed out, difficult terrain around that as well. There needs to be an easily defensible way in and out through that terrain which bottlenecks attackers. Eventually you will run out of boiling oil and wood for arrows.
Speaking of arrows, while you can use alt crafting methods, you do need bird feathers...so...hey, going to need a lot of birds. Which you also have to feed. But they will be delicious later.
YES, they can be self sufficient, if they build a wall around everything, including food production fields.
Take the Anime Attack on Titan as an example.
In Attack on Titan, only scouts go outside. Their purpose is not to trade or gather food, but to "scout" and kill some big guys.
The city feeds themselves. They are believed to be what is left of humanity. Perhaps an apocalypse is a good motivation to build a self sufficient city, no matter the timeline.
Here is some mathematics on how many land a city needs to support its population. https://www.youtube.com/watch?v=uQ_VfizNaJQ (it is about Attack on Titan too)
