Feasibility of entirely self-supporting fortress

Question

In a Medieval Fantasy style world, I'd like to have a city/fortress that is impossible to besiege, because they can produce all of the food they need within the strong city walls. However, I'm not sure that this is feasible, as with more land to produce food you need more space, therefore more people to defend the walls, therefore more food, therefore more space, ad infinitum.

I am somewhat familiar with ancient through medieval military tactics and sieges, but not at all familiar with how farming technology may have worked. Is there a semi-plausible way for a fortress city to provide enough food for those defending the walls, so that they could never be starved out? I'm thinking about locating the fort near or on a mountain, which when the rains come works to funnel the water into giant cisterns under the city that provide enough clean water until the next rain. The only problem is food.

I've considered networks of tunnels where they grow food, but that would lack sunlight. Animals won't work because they take too long and use many resources before they provide a return, I know this at least from raising them myself in the modern day. Is there a specific, highly efficient food source that might work best? How could they maximize food production to make this possible?

Answer 1

Island Fortress

With an island, you have natural fortifications and it is far easier (at least prior to the age of steam and ironclads) to defend than to attack. Have one natural, easily defended harbor, with the rest of the island having sheer cliffs going deep down into the sea.

The middle of the island is fertile land for farming. Catch rain for drinking. In addition to the farming, you can catch fish for some good protein without needing the large amount of land needed for cattle/sheep/etc.

Answer 2

Mathematically

Let's assume that the actual farming is done "by magic" for the moment. A relevant question from Reddit suggests that a meat-inclusive diet needs about 0.5 hectares/person. Assuming that a wall is defended with one person per metre, this farmland is 5km deep behind the wall. So a "city" of two walls with 10km of farms between and impassable mountains on the ends fits the criteria. However, increasing the size helps. If the city is circular, doubling the radius will double the amount of wall and quadruple the arable land. This can go on for as much ground as you can claim.

• meaning, "therefore more space, ad infinitum." is not correct, surface area grows proportinal to square of size, perimeter linearly. 1 percent of people defending wall and all other numbers the same, diameter of your city is about 1000 km, population 314 million. Congratulation, you got an achievement - invention of a country/state. All of them protect wall, 100 times less surface, 100km diameter, 3.14 million population. But due size it will be a poor forthress.

Your proportions can be improved vastly by using a natural wall. A narrow pass through harsh mountains allows you to have a 1km wall in the pass do the work of a 100km wall on the plain (or better). "Shangri-La" can be a fertile valley among mountains, possibly with hot springs.

Practically

After a certain point, your wall (or parts thereof) is a very long way from the city, and so you build outposts. And support depots for farmers to live. And then you realise that you're building this:

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In other settings, you can claim that fusion-powered aquaponics/hydroponics systems do the work and scale vertically. Or that magical effects can grow/create food.

Answer 3

The short answer is: it depends on many variables. But YES, this is possible.

The long answer requires at least some variables be addressed.

1. What magic, if any, is in play? Are these all humans, or are other races involved? Assumptions: no magic, only humans.
2. What technology level are we at? Advances in farming make it easier to support larger populations with less land. Likewise, advances in siegecraft make it easier to break a wall, vs advances in construction to make a stronger wall, and to make it easier to defend a wall with fewer soldiers. Also relevant would be medical technology for the castle (to prevent plague, either natural or from the besieging army). Assumptions: high middle ages, European technology. Say, 1250AD.
3. How good is the area for farming? How "rich" is the soil, is there enough moisture, how long is the growing season, etc. Assumptions: the defenders are using multiple crops to prevent soil depletion, and are in England-like conditions. They average 10 bushels of grain per acre (a real amount for the time and location). We will assume no droughts or other uncontrollable factors occur during the siege. We are vastly simplifying this section, just go with it.
4. How long are we dragging the siege out? We will say the fortress needs to sustain itself for 25 years. This is an absurd siege length, but we're going with it anyway. We will further assume they have stockpiles of leather, metal, wood, reeds for gambeson, etc to last that time. Firewood is being deliberately left out of our calculations for simplicity.
5. What is the morale of the defenders? Even if the castle can hold out indefinitely, the individuals within may not be willing to. Assumptions: the defenders are highly motivated to never give in. The besieging army takes no quarter and wants to wipe out all the heretics in the defending castle, and all the defenders know this.

For a location, we'll say a plateau peninsula with a narrow crossing attaching it to the mainland, ocean on three sides, and high steep cliffs from ocean level to the plateau. This means there is no landing for boats. We'll say the land entrance is sloped upward to give advantage for the defenders, and the ground in the pass is very rocky, making it hard to tunnel underneath. The wall at the pass is 4 meters thick (12 feet) and 12 meters tall (39 feet), with arrow slits, rounded towers, and all the best innovations for this time period. Conceivably, such a location and fortification would be nigh impossible to take directly (remember: no cannons for this time period).

We will further say the attackers are unwilling or unable to use biological attacks, or the defenders for some reason are capable of neutralizing them. In other words, no rotting cows hurled into the fortress to spread disease.

Most historical "castles" were small affairs. We're assuming this is a proper military fort, not the fortified manor of a knight or minor lord. Let's set the garrison at 800 fighting men (including men at arms and knights), which is quite large for the time period but within the realm of possibility (2000 men is probably the largest garrison of any castle in the period).

Roughly 300 peasants, craftsmen, clergy, and other supporters were required to support one knight's family in England during this time period. A fort with 800 men would likely have 50-70 knights. Let's say 750 soldiers and 50 knights. That's 15,000 people just for the knights, plus the garrison, for 15,800 so far. Then we need additional people to support the 750 men at arms.

That 300 number is derived from averages in England during the time period for knights living in separate manors. We can assume we'll need fewer craftsmen, for example, since all the knights are in one location instead of spread out. I can't find good numbers for this, so we are going to say that 15,200 people can support the entire 800 man garrison. That gives us a total population of 16,000 people behind the walls for the 25 year siege.

For reference, 16,000 people is nearly the size of London in 1200AD. If we say our numbers (the 300 per knight) were bad, and halve the total to 8,000 people (7,200 support and 800 garrison), that is still a large city by the standards of 1250AD Europe.

Now, it takes an average of 20 bushels of grain to support a person for a year. With our average of 10 bushels per acre, that means 2 acres per person. With 8,000 people, that's 16,000 acres being actively farmed. With crop rotation, some of the farmland is not being farmed at any given time. If 2/3 are being farmed and 1/3 is left fallow, we're at 24,000 acres of land necessary to support the people, plus whatever land they need to actually live on, have shops, barracks, etc. We are ignoring that land for simplicity.

That does not include the resources for maintaining horses for the soldiers. Other livestock could graze on the unplanted land, but war horses can't be left out to roam around, since they need to be available for soldiers. And make no mistake, they will be needed for patrols: our assumed location may be impossible to reach by boat, but the defenders will have to patrol their cliffs for any attackers trying to scale them. There are many historical examples of castles falling because small groups made it in past "impossible" mountains or cliffs.

24,000 acres is about 97 square kilometers, or 37.5 square miles. Double that number if you want the original 16,000 people, to 194 square kilometers or 75 square miles. For reference, 97 square kilometers is about the size of Disney World. 194 square kilometers works out to be about 4/5ths the size of modern Edinburgh, Scotland.

Can you enclose that much land, with that many people, behind a wall and maintain it for 25 years? Absolutely, given the physical features described above. But it is not normal for any civilization or time period I know of. It would be nigh impossible to find that perfect location with enough arable land to pull it off. If you reduce the numbers involved, perhaps making this a fort with only 100 defenders and their supporters, you can drop the size considerably, making it easier to find such a location. If you conscript the farmers to hold the wall during active fighting, you could maintain a longer wall.

Furthermore, rationing would reduce the food needed from 20 bushels per person per year to 15 bushels. Anything less than that is going to cause problems if maintained over such a long time period. If the fort starts the siege with existing food storage, they could be producing less food than they need, but coast through with the storage; this puts a timer on how long they last, but a sufficient amount of storage with close enough food production means they could last 25 years.

Sidenote: This assumes no additional refugees fled to the fortress and need to be fed while being unable to productively do anything. Or that some of the land behind the walls was unused, and all the refugees are able to become productive on the unused land. Or that enough casualties are taken early on that refugees take over farming areas previously farmed by now-dead people.

Sidenote 2: See Attack on Titan for a fictional, self-sustaining mega fortress. Some, though certainly not all, of that show could get your mind thinking of different scenarios and how to address them.

Answer 4

Concentration of force makes this a bad idea

There is an age old military doctrine called concentration of force that establishes that the effectiveness of a military force is not how big it is, but how concentrated it is. A group of 100 soldiers moving as a solid unit can overwhelm 1000 soldiers spread out over a large area because they will have the advantage of numbers for each skirmish.

So instead of asking could it happen, you should look at why it did not happen more. History is full of examples of civilizations trying to build long garrisons like the Great Wall of China, the Maginot Line, and Hadrian's Wall where nations tried to block off massive territories against invasion. The problem with this technique is that it always failed because a defensive position is only as good as its weakest point. A single watchman can be bribed or a single blind spot be identified as a point to attack completely nullifying any advantage that the wall would otherwise give you.

Most military engineers have understood this principle since at least the time of the Roman Empire. The reason most historical castles were kept so small was because they concentrated your defensive force.

Example:

If you have a 13.5km circular wall surrounding about 3600 acres of good farmland maintained by ox gangs, this will give you a town with about 720 households with 240 households committed to maintaining the farmlands, and 480 households you can commit to other things like blacksmithing, carpentry, etc. For population demographics, this means you have about 720 men who are "heads of household", their 720 wives and their 3240 living children (some of which may be adults) for a total population of ~4680.

In general, each household has an average of 1 man of military age (either a younger father or older son). That said, arming professional soldiers is expensive. Using the Hide system we can assume that this land would support ~30 professional soldiers (aka:knights) but you could commit up to ~480 additional militia to guarding the walls in times of need and still keep the farms running. This gives you what would normally be considered a sizeable garrison for a castle.

But, the problem here is that your town would be at least 4.3km across at its widest point. That would take about an hour for your whole garrison to be moved to any one point on the wall to concentrate your defenses enough to repel a large scale attack. Furthermore, a feigned attack can be used to draw defenders away from one part of the wall while your main army then goes after a now undefended section several kilometers away from where you just sent the bulk of your garrison.

Castles were the solution to this. They were small defensible locations that you could move your knights and militia into while you send some or all of your "unneed" people to nearby communities as refugees until the siege is over. As long as you have a castle, the enemy army is encouraged not to keep advancing because your garrison would cut off its supply lines, but if they attack it, they will be attacking a concentrated force of defenders.

Your average medieval castle had a permanent garrison of 30 professional soldiers, and had about 400 meters of outer walls (making no two places more than 150m appart), since most healthy people can sprint about 7 m/sec over short distances like this, it means you can measure the time it takes to reinforce a part of the wall in seconds rather than an hour. Also, they could generally house a lot more than their standing garrison; so, many of your militia could be called up to help hold the castle. Because the castle is small, but can hold the same garrison as your giant walled off farmlands, you can always fight concentrated when defending against an attack.

Every solution mentioned so far either ignores concentration of force with ideas like giant walls many km long or they give examples of using natural terrain to put a normal sized castle between your farms and the most likely route of attack. In reality though, no cliff or wall is impossible to overcome if the attackers are determined enough unless you actually defend it; so, it is in most cases just a better tactic to abandon your farms than to try to defend them unless you have some seriously impassable natural defenses.

Answer 5

A fortress made to defeat enemy siege attempts would usually have large stores of food, restocked while at peace so it will need to grow only limited amount of vegetables etc to provide fresh, vitamin rich food. Not many armies would be able to maintain siege for extended periods after all. Mountain or hill side fortress may use terraced farming to utilize as much space as possible if you have enough rain to sustain such farming.

The biggest problem is you need to balance two things - the shorter the wall, the easier is it to defend while you need a lot of land for farming which increase the length of the wall. And as wall become longer, it may be impossible to defend.

If enemy is truly ingenious, no fortress stand a chance. Alexander the Great managed to capture essentially what you want - mountain plateau fortress that had single access point, ample supplies and farms by sending a group of climbers who scaled the side of the plateau and appeared on the highest point. The defenders, facing what they considered an impossibility, surrendered immediately.

By the way a city on a shore is way more suited to be (almost) impregnable to siege. Obviously as long as enemy navy is not massively superior. Just superior would not suffice as weather can still permit to slip past blockade and bring in supplies.

Answer 6

Why wall a bunch of farms and houses when you can just make a big gate and call it a fortress?

Rather than the fortress itself being self-suficient, it protected the only path to a small fertile valley in the middle of the mountains. There might be other paths to the valley, but if you are not a mountain goat, you probably cannot use it. In the valley the actual city is located, as well as farms for food production, a small managed wood to ensure a continous supply of wood, quarries to make actual useful stone, mines to get the iron to repair armours and weapons and surely the most important resource of them all: a stable population, to supply soldiers to protect the fortress, engineers to repair it after attempts to breach the walls, craftmen to produce other essentials that nobody cares about.

This technically works around the problem rather than adressing it, but this solves not only the food issue, but most issues that would afflict a siege short of a century. If you are going to fantastical siege times - say a thousands years - other issues would appear, such as the fortress eroding and the engineers cannot repair it while the besiegers make it rain arrow on them, or the valley population becoming inbred to the point of disfunctionally.

Answer 7

Underground homes

The basis of your problem is that people need space to live, so does food need space to grow, we cannot grow plants underground (maybe except for mushroom), so why not make a city where the general population lives underground and the ground itself is used for agriculture. In this case, the homes need to build quite deep so as to provide a good amount of topsoil for plants to grow and require lighting, air supply, and other essentials, but it should work. (The reason people shifted underground was the harsh weather in the first place, but they found out it is a great war tactic too.)

Massive Food stores Along with agriculture, this city should have massive grain storage facilities, to help during sieges.

Vegetarian diet It will be important for people to have a vegetarian diet so that the overall food requirement for cattle can be minimized.

Tunnel networks for sabotage Surviving a siege is not the only important thing, It's also good to have a network of secret tunnels to sabotage enemy plans/supplies to make them leave early. Also to acquire essential items not available in the city. (Story suspense: someone betrayed the city and revealed the secret tunnels to the enemy.)

Answer 8

If you are willing to get a bit flexible on what you count as a fortress sure.

For practical purposes, a fortress can be any defensible position that is at least partially man-made. Now, traditionally terrain has had some impact. Building your fort or castle on a hill has been popular since pretty much forever.

Now, take terrain advantage up several levels. Surround a city with impassable mountains on three sides, build a wall on the fourth, and you have something no invading army can feasibly get to. You can probably do some works to make any mountain passes impassable if that's your goal, and you have a city that can produce all the food it needs, along with a minimal area that actually needs defending.

Answer 9

Have your people live on a peninsula on cliffs.

You'd only have to fortify the entrance area. If the peninsula is large enough you have the room you need for farms, homes and fortifications. And if its high up enough on those cliffs it can even protect from boat attacks. And the entrance area could be on high ground making it even more difficult for attackers.

Several answers, including mine, in the question How would a society isolated by monster attacks get food? build on this idea. Take a look for some cool pics and drawings that explain further.

Answer 10

I believe such a fortress could technically be possible, but it would be difficult. The fortress would need to grow food that requires very little to grow like mushrooms, which can be grown in small damp spaces with little light in compost/moist environments. Certain types of algae can also be harvested from unfiltered water as a source of nutrition. These two items can be the staple foods of your fortress with other foods being held in reserves or obtained in times of peace. In open areas, certain 'weeds' like dandelions can be used as food as well as medicine while growing easily in most locations. If you have water that you can keep with a pH between 6 to 7, you can grow wheat using aquaponics. You can also use this method to grow tomatoes, lettuce, peppers, onions, microgreens, onions, and dwarf citrus fruit for your fortress.

Answer 11

Food in tunnels

If you'll accept a bit of creative difference in your world, you can still use the caves. First it can be a prime nesting ground for creatures like bats, making it also a siurce of food. But the droppings can be ysed by fungi for example. In my knowledge most (none?) aren't edible for humans, but here you can use the freative difference.

Algea can also live in the darkest depths of the caves. This is of course a much less effective method to produce energy and grow, but it isn't clear to me how. Likely heat can be used if the caves go deep enough in the crust. Also here you can give the creative difference a whirl and make them edible and possibly fast growing to be suitable for a sort of farming.

Natural formations

There's loads of ways that you can make a fortress, bit most is done by using the landscape. Rivers, mountains and gorges can easily be implemented. A mountain range can be difficult and dangerous to teaverse for an army. Even if a small group could go through, it doesn't allow an army. This greatly reduces the amount of people needed to defend that side. This can allow for much greater parts of land to be added inside the fortress.

Answer 12

If your fantasy world has a little magic you could use it to grow an enormous and bountiful apple tree, then build your castle walls around its outermost branches.

What I was trying to say in this brief answer is that if the author is crafting a fantasy world, complete with magic, then that magic could easily provide self-sufficiency on a much smaller scale than would work under the real-world's physical laws. If for example, you enchanted an apple tree such that it grew to skyscraper height and if its easily climbable branches held fruit at every level, then that one tree could feed your entire castle population. It might also disturb the local wind patterns such that enough dew gathers on its many, many leaves to satisfy the castle's need for clean water. This single tree, standing alone on an open plain could by itself support a self-sufficient colony. With such a blessing at the core of their castle, the population's only real challenge would be to build a strong enough wall all the way around the tree's lowest branches, so that invading armies cannot also enjoy the mighty tree's fruit.

Answer 13

Encircle an EXTINCT Volcanic Mountain with the Wall

You build your city around a single old volcanic mountain on a plain. City walls surround the mountain with some flat surface in between. City is built on the higher and rougher parts of the mountain. Farming is done at the flat plain level. And that is where your walls are located. The volcano provided the land with highly nutritious soil. Mining is also done on the mountain providing the city with metals it needs. The mountain also has water springs flowing out of it which provides the city with the water it needs.

With this setup;

1. The besiegers cannot contaminate the water city uses.
2. Cannot easily use their medieval weapons to attack buildings inside the city as buildings are high on the ground and has farmlands between themselves and walls.
3. Metals can be mined in the walls, food can be grown in the walls, water spring is in the walls.

Without any major distruptions like no snowfall, drought, a hole in the wall etc. the city can be self sufficient for very long times.

Yet you have to consider that besiegers will try to dig under the city walls. Walls has to be deep as much as it has to be tall. Even that, given long enough time, might not be enough to stop the besiegers. But the city will be self supporting.