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Question

Let's say we want to build the strongest possible fortressÃ‚Â¹ in a medieval (1300 CE) setting while having access to any modern technology and material.

Said technology can only be used during the construction of the fortress and not afterwards, so e.g. a solar powered device is allowed to continue functioning after the construction is complete, but the denizens of the fortress will not be able to repair it in case of damage.

Ã‚Â¹This fortress will have to endure:

Battering rams on the gate

Balistas, catapults and trebuchets. No cannons.

Starvation by means of cutting out supply lines.

Possibly, ladders and siege towers. (I say "possibly" because it would be a nice addition, but as long as the other 3 problems are covered, this can be handled by the soldiers.)

It is assumed that the inhabitants will learn perfectly how to operate every device/building/etc., but will not be able to understand how/why they work.

This means they won't be able to repair or replicate anything beyond their level of technology. For example, if a carbon fiber wall gets damaged, they will repair it with stones/bricks/concrete.

Any location is allowed, provided it is suitable for a medieval population (e.g. an underwater city would be infeasible). Population: as many people as possible as long as it doesn't compromise security and provisions. The more, the better.

What are the materials and techniques best suited to withstand the problems listed?

If "impregnable" is impossible, then "as strong/durable as possible".

Bonus points if the answer manages to give a great solution to the problem without needing overly complicated technology to keep working (e.g. by focusing on the materials and construction techniques rather than on particular devices)

No way. There is no wall on this world which cannot be undermined, there is no wall on this world which cannot be overtaken by a determined enemy. The essence of the problem is that the fortress has limited resources, whereas the enemy can bring unlimited resources. Any fortress can be taken; the purpose of a fortress is to compel the enemy to allocate so many resources to its siege that his strength will be sapped elsewhere. — 1 hour ago
Will just the structure be modern, or can we store modern food in modern containers? modern medicines, etc? -- Also, very important: 1) how long would a siege last, and 2) how many occupants must be provisioned and 3) must there be some offensive capability/ sortie-possibility and 4) must there be some other feature like the possibility to take in friendlies etc. — 1 hour ago
I would very seriously advise you to read the Emberverse books but the short version goes like this; use modern materials and techniques to recreate old designs that worked. — 36 mins ago
@bukwyrm Anything that doesn't need modern knowledge to be maintained is OK (so food is ok). 1) How long did sieges last in the 1200s? 2) As many as possible without compromising security and provisions. 3-4) Must function the same as pretty much any fortress from the middle ages. So, yes to all of these — 12 mins ago

Battle 3698 · Answer 1 · 2018-10-10 09:51:29Z

up vote
3
down vote

The equivalent to fortresses in modern times are bunkers. Dig down deep, have a huge sturdy steel gate in a tunnel and that will be the only vulnerable spot - which should be indestructible for medieval level of technology. Consider creating a huge hole in front of the gate with water at the bottom. In order to have a battering ram roll to the gate, which wouldn't be effective anyway, they'd need to construct a lot in order to even get there.

Also they wouldn't be able to dig through mountains and ferroconcrete in any reasonable time and expense, so the gate is the only way in.

answered 2 hours ago

Battle

3698

1

What about supply lines getting cut off?
â€“Â Hankrecords
2 hours ago

Bunkers today already have sufficient supplies to last for many years for many people. You just have to store food which lasts long. Otherwise the answer is the same as for today's bunkers. They are made for nuclear fallouts, assuming you cannot go outside for a long period of time - supply lines are not intended to be had in the first place. Otherwise I can't think of surface structures which could withstand siege for long periods of time...
â€“Â Battle
2 hours ago

2

And what would be the use of a deep bunker in medieval times? All the enemy has to do is to park some soldiers in front of that steel door, and your army is trapped. Wars are not fought for points, they are fought for control over territory and resources. If you are willing to trap your army in a concrete cage with an easily guarded steel door the enemy will be happy to oblige.
â€“Â AlexP
1 hour ago

2

In the words of General Callus Tacticus if the enemy has an impregnable stronghold, see he stays there, your bunker sounds like a perfect candidate for this approach.
â€“Â Separatrix
1 hour ago

Whether trapped or not, @Hankrecords has a point - the technology of the day barely allowed for the natural cold weather and salting to allow some foods to be stored over winter, there were no "long life foods" in the modern sense. So without adding preparation by using modern methods (eg canning, freeze drying) to lay in a stockpile of said long life foods, this answer is failing to address one of the OP's key criteria.
â€“Â KerrAvon2055
1 hour ago

Â |Â
show 1 more comment

Separatrix 69.2k30161272 · Answer 2 · 2018-10-10 11:37:08Z

Colditz Castle

An impregnable fortress. An inescapable prison.

This is the problem you risk, if you make a fortress that's truly impregnable, you risk it being the prison you spend the rest of your life in. One of the key features of castles is the ability to counter attack. Whether to sally forth with troops, or simply to shoot from the relative safety of the battlements a castle is more than just a reinforced concrete box.

Why a castle?

Castles are a statement of control as well as a tool of war, you see it dominating a landscape and it tells the world that there's someone here willing to put up a fight to keep a region. It allows a smaller force to hold a territory against much larger numbers until reinforcements can be gathered. It also acts as garrison, armoury, residence, administrative centre, and food store. Castles are not the purely functional buildings that modern bunkers tend to be.

A castle didn't have to hold out forever when under siege, it just had to hold out for long enough. Directly assaulting a castle is fairly rare, it's expensive and risky. Far cheaper to sit out the siege, which could last years (the siege of Donnington castle lasted from July 1644 to April 1646). If your attackers are in a hurry and going for the attack you're probably in luck as defenders, castles were good and the ones that survive intact into the modern age were the very best. Dover castle lasted long enough to be hardened against cannon fire with the best technology of the age.

So what to change?

One of the greatest vulnerabilities of a castle was being undermined. This could bring down even the strongest tower. What we're going to do to compensate for this is dig down a couple of storeys, drill down and put in reinforced concreted foundations. While we're down here we'll also create some basements with water storage tanks and cool food storage areas all surrounded by double layered reinforced concrete walls, infilled with soil and rubble, similar to the main castle structure but underground and concrete. We may leave some secret tunnels down here just in case we need to make a discrete exit. Ideally there will also be a well or similar underground fresh water supply.

For the superstructure I'm not going to change much, the people of the day knew what they were doing and most castles would easily resist a direct assault on the walls. This leaves them in a good position to repair any damage that may be done during the average battle.

Inside the main outer door of the barbican we're going to build a bank vault type door, a simple conical section, smooth on the outside, it can only be controlled from the inside. The corridor between the outer door and this second door is a straight up kill zone, oil holes in the ceiling, spear holes in the walls all the usual "kill anyone in here" stuff, nothing special and fairly standard for the day.

In summary

Don't underestimate how good castle builders were at their job. Most castles never fell to assault, if they fell it was normally a surrender after a siege. The real change is better understanding of sanitation and food storage. Castles could survive sieges for years as they were.

+1 for the sanitation. If there is one modern thing you can add to a medieval castle, it should be the plumbing — 2 mins ago

Guran 2,9131124 · Answer 3 · 2018-10-10 11:47:20Z

Resist battering rams on the gate

The best defence againt a battering ram is of course to make it really difficult to get a ram up to the gate, but that does not depend on tech level.
What you can do is to make a solid steel door. (Easy with modern tech, well understood but very very expensive for a person in the middel ages)

Also construct hinges and fittings carefully. Place some cushioning material (rubber will do nicely) behind the gate post to absorb impact from the ram.

Resist Balistas, catapults and trebuchets.

You can certainly build strong walls with reinforced concrete. Also add steel plates and a crumble zone in front of the walls to greatly reduce impact from any boulders hurled your way.

Resist Starvation by means of cutting out supply lines.

Modern tech would not add much here. The key is to have a reliable well inside your fortress and stock upp on food beforehand.
However, you could definitely build some better facilities for hygiene and sanitation, reducing the risk of your soldiers succumbing to diseases.

Resist ladders and siege towers.

Using reinforced concrete instead of stone makes it easier to build really high walls, which naturally improves your defence against ladders and towers.
Also, you can build some nice defensive constructions, having protruding structures high up from which your defenders can easily cover the walls with arrows (and nastier stuff) making any attack over the walls a suicide mission.

Resist tunnels (bonus)

The favourite means of breaching a castle wall was to tunnel under it and set fire to the (wooden) support beams.
With modern equipment and machinery, you can extend your wall all the way down to bedrock, or at least well below groundwater level, making tunneling very hard.

Put the gate at the top of a slope. The ram's own weight will pull it backwards. Problem solved — 1 min ago

ColonelPanic 1,909110 · Answer 4 · 2018-10-10 11:13:12Z

The secret lies in Agriculture and Sanitation

Most medieval constructions before the advent of cannon/gunpowder were actually sufficient to prevent entry against a determined adversary. Most of these sieges ended through starvation, disease, an insider threat, or a counterattack by the defender's allies.

First of all, let's just make sure that your castle is truly impregnable. To do this, we are interested in two things: height (which means range) and the impregnability of the construction materials. We make our walls higher than any siege engine can launch an object. This will also prevent siege towers from being a factor. Now we can't have outside fire or disease (or deadly projectiles) launched in to our city. We also want to place footers under our walls deeper than can be possibly undermined. Current technology allows for both of these feats. The wall construction will be made of a composite lasting the centuries... likely reinforced concrete, layered with impact absorbers, covered with a durable, yet thin, metal. This will absorb anything thrown against it.

Now, the gate/entry way is the weakest physical assault point. This means that we will want multiple layers of security here and a precarious approach. A narrow, and easily protected, walkway covered by multiple towers on our skyscraper walls will do the trick, we'll just make several of these to be sure (and remove the idea that one person could open the single gate to our city. Also, a system could be developed where only one of the three inrow city gates can be opened at a time, along with much more secure (while still being simple) locking mechanisms.

So what's left... fire? All structures inside of our city are made of steel reinforced concrete. Disease? Sanitation is the name of the game here for the most part and a sewage system is put in place to deal with this. Romans did it with older technology, we can do it as well. Fresh water is provided by placing the castle over a large reservoir and drilling to this point. Modern technology isn't needed to lift water out of this in the event that pumps break. Impart as much medical knowledge as feasible (just the idea of infection and how to prevent it should safe thousands).

Now we're neatly sealed up inside of our city, with plenty of water and housing. We need food. And a way to always have food. You simply enlarge the area enclosed by your massive fortress walls to grow ample food to supply the city. It is likely best to make this area dispersed and divided to prevent disease/fire from spreading among the farms. The area required can be greatly reduced over typical medieval farming practices due to introducing modern agricultural techniques and grains. We also introduce modern grain storage techniques which allows for massive hoarding of food.

As for weapons to defend ourselves, sure, you could introduce all manner of fancy modern weaponry, but given our height and armored supremacy, traditional siege weapons mounted 100m off of the ground can outreach any adversary, if they even bother to assault you, which they never would.

Daniel 2,636714 · Answer 5 · 2018-10-10 12:09:08Z

Build one of these AA-Towers the Nazis build in WW2. The Allied Forces tried to destroy them (with explosives from the inside) and ultimately gave up after having to use 40 tons of TNT on one of them in Berlin.

enter image description here From KMJ, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=968914

It sure can withstand anything any medieval weapon has to offer. DonÃ‚Â´t forget to bore a well in the middle!

Battle 3698 · Answer 6 · 2018-10-10 09:51:29Z

up vote
3
down vote

The equivalent to fortresses in modern times are bunkers. Dig down deep, have a huge sturdy steel gate in a tunnel and that will be the only vulnerable spot - which should be indestructible for medieval level of technology. Consider creating a huge hole in front of the gate with water at the bottom. In order to have a battering ram roll to the gate, which wouldn't be effective anyway, they'd need to construct a lot in order to even get there.

Also they wouldn't be able to dig through mountains and ferroconcrete in any reasonable time and expense, so the gate is the only way in.

answered 2 hours ago

Battle

3698

1

What about supply lines getting cut off?
â€“Â Hankrecords
2 hours ago

Bunkers today already have sufficient supplies to last for many years for many people. You just have to store food which lasts long. Otherwise the answer is the same as for today's bunkers. They are made for nuclear fallouts, assuming you cannot go outside for a long period of time - supply lines are not intended to be had in the first place. Otherwise I can't think of surface structures which could withstand siege for long periods of time...
â€“Â Battle
2 hours ago

2

And what would be the use of a deep bunker in medieval times? All the enemy has to do is to park some soldiers in front of that steel door, and your army is trapped. Wars are not fought for points, they are fought for control over territory and resources. If you are willing to trap your army in a concrete cage with an easily guarded steel door the enemy will be happy to oblige.
â€“Â AlexP
1 hour ago

2

In the words of General Callus Tacticus if the enemy has an impregnable stronghold, see he stays there, your bunker sounds like a perfect candidate for this approach.
â€“Â Separatrix
1 hour ago

Whether trapped or not, @Hankrecords has a point - the technology of the day barely allowed for the natural cold weather and salting to allow some foods to be stored over winter, there were no "long life foods" in the modern sense. So without adding preparation by using modern methods (eg canning, freeze drying) to lay in a stockpile of said long life foods, this answer is failing to address one of the OP's key criteria.
â€“Â KerrAvon2055
1 hour ago

Â |Â
show 1 more comment

Separatrix 69.2k30161272 · Answer 7 · 2018-10-10 11:37:08Z

Colditz Castle

An impregnable fortress. An inescapable prison.

This is the problem you risk, if you make a fortress that's truly impregnable, you risk it being the prison you spend the rest of your life in. One of the key features of castles is the ability to counter attack. Whether to sally forth with troops, or simply to shoot from the relative safety of the battlements a castle is more than just a reinforced concrete box.

Why a castle?

Castles are a statement of control as well as a tool of war, you see it dominating a landscape and it tells the world that there's someone here willing to put up a fight to keep a region. It allows a smaller force to hold a territory against much larger numbers until reinforcements can be gathered. It also acts as garrison, armoury, residence, administrative centre, and food store. Castles are not the purely functional buildings that modern bunkers tend to be.

A castle didn't have to hold out forever when under siege, it just had to hold out for long enough. Directly assaulting a castle is fairly rare, it's expensive and risky. Far cheaper to sit out the siege, which could last years (the siege of Donnington castle lasted from July 1644 to April 1646). If your attackers are in a hurry and going for the attack you're probably in luck as defenders, castles were good and the ones that survive intact into the modern age were the very best. Dover castle lasted long enough to be hardened against cannon fire with the best technology of the age.

So what to change?

One of the greatest vulnerabilities of a castle was being undermined. This could bring down even the strongest tower. What we're going to do to compensate for this is dig down a couple of storeys, drill down and put in reinforced concreted foundations. While we're down here we'll also create some basements with water storage tanks and cool food storage areas all surrounded by double layered reinforced concrete walls, infilled with soil and rubble, similar to the main castle structure but underground and concrete. We may leave some secret tunnels down here just in case we need to make a discrete exit. Ideally there will also be a well or similar underground fresh water supply.

For the superstructure I'm not going to change much, the people of the day knew what they were doing and most castles would easily resist a direct assault on the walls. This leaves them in a good position to repair any damage that may be done during the average battle.

Inside the main outer door of the barbican we're going to build a bank vault type door, a simple conical section, smooth on the outside, it can only be controlled from the inside. The corridor between the outer door and this second door is a straight up kill zone, oil holes in the ceiling, spear holes in the walls all the usual "kill anyone in here" stuff, nothing special and fairly standard for the day.

In summary

Don't underestimate how good castle builders were at their job. Most castles never fell to assault, if they fell it was normally a surrender after a siege. The real change is better understanding of sanitation and food storage. Castles could survive sieges for years as they were.

+1 for the sanitation. If there is one modern thing you can add to a medieval castle, it should be the plumbing — 2 mins ago

Guran 2,9131124 · Answer 8 · 2018-10-10 11:47:20Z

Resist battering rams on the gate

The best defence againt a battering ram is of course to make it really difficult to get a ram up to the gate, but that does not depend on tech level.
What you can do is to make a solid steel door. (Easy with modern tech, well understood but very very expensive for a person in the middel ages)

Also construct hinges and fittings carefully. Place some cushioning material (rubber will do nicely) behind the gate post to absorb impact from the ram.

Resist Balistas, catapults and trebuchets.

You can certainly build strong walls with reinforced concrete. Also add steel plates and a crumble zone in front of the walls to greatly reduce impact from any boulders hurled your way.

Resist Starvation by means of cutting out supply lines.

Modern tech would not add much here. The key is to have a reliable well inside your fortress and stock upp on food beforehand.
However, you could definitely build some better facilities for hygiene and sanitation, reducing the risk of your soldiers succumbing to diseases.

Resist ladders and siege towers.

Using reinforced concrete instead of stone makes it easier to build really high walls, which naturally improves your defence against ladders and towers.
Also, you can build some nice defensive constructions, having protruding structures high up from which your defenders can easily cover the walls with arrows (and nastier stuff) making any attack over the walls a suicide mission.

Resist tunnels (bonus)

The favourite means of breaching a castle wall was to tunnel under it and set fire to the (wooden) support beams.
With modern equipment and machinery, you can extend your wall all the way down to bedrock, or at least well below groundwater level, making tunneling very hard.

Put the gate at the top of a slope. The ram's own weight will pull it backwards. Problem solved — 1 min ago

ColonelPanic 1,909110 · Answer 9 · 2018-10-10 11:13:12Z

The secret lies in Agriculture and Sanitation

Most medieval constructions before the advent of cannon/gunpowder were actually sufficient to prevent entry against a determined adversary. Most of these sieges ended through starvation, disease, an insider threat, or a counterattack by the defender's allies.

First of all, let's just make sure that your castle is truly impregnable. To do this, we are interested in two things: height (which means range) and the impregnability of the construction materials. We make our walls higher than any siege engine can launch an object. This will also prevent siege towers from being a factor. Now we can't have outside fire or disease (or deadly projectiles) launched in to our city. We also want to place footers under our walls deeper than can be possibly undermined. Current technology allows for both of these feats. The wall construction will be made of a composite lasting the centuries... likely reinforced concrete, layered with impact absorbers, covered with a durable, yet thin, metal. This will absorb anything thrown against it.

Now, the gate/entry way is the weakest physical assault point. This means that we will want multiple layers of security here and a precarious approach. A narrow, and easily protected, walkway covered by multiple towers on our skyscraper walls will do the trick, we'll just make several of these to be sure (and remove the idea that one person could open the single gate to our city. Also, a system could be developed where only one of the three inrow city gates can be opened at a time, along with much more secure (while still being simple) locking mechanisms.

So what's left... fire? All structures inside of our city are made of steel reinforced concrete. Disease? Sanitation is the name of the game here for the most part and a sewage system is put in place to deal with this. Romans did it with older technology, we can do it as well. Fresh water is provided by placing the castle over a large reservoir and drilling to this point. Modern technology isn't needed to lift water out of this in the event that pumps break. Impart as much medical knowledge as feasible (just the idea of infection and how to prevent it should safe thousands).

Now we're neatly sealed up inside of our city, with plenty of water and housing. We need food. And a way to always have food. You simply enlarge the area enclosed by your massive fortress walls to grow ample food to supply the city. It is likely best to make this area dispersed and divided to prevent disease/fire from spreading among the farms. The area required can be greatly reduced over typical medieval farming practices due to introducing modern agricultural techniques and grains. We also introduce modern grain storage techniques which allows for massive hoarding of food.

As for weapons to defend ourselves, sure, you could introduce all manner of fancy modern weaponry, but given our height and armored supremacy, traditional siege weapons mounted 100m off of the ground can outreach any adversary, if they even bother to assault you, which they never would.

Daniel 2,636714 · Answer 10 · 2018-10-10 12:09:08Z

Build one of these AA-Towers the Nazis build in WW2. The Allied Forces tried to destroy them (with explosives from the inside) and ultimately gave up after having to use 40 tons of TNT on one of them in Berlin.

enter image description here From KMJ, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=968914

It sure can withstand anything any medieval weapon has to offer. DonÃ‚Â´t forget to bore a well in the middle!

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Build an impregnable fortress in the middle ages with modern technology

5 Answers 5

Colditz Castle

An impregnable fortress. An inescapable prison.

Why a castle?

So what to change?

In summary

Resist battering rams on the gate

Resist Balistas, catapults and trebuchets.

Resist Starvation by means of cutting out supply lines.

Resist ladders and siege towers.

Resist tunnels (bonus)

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5 Answers 5

5 Answers 5

Colditz Castle

An impregnable fortress. An inescapable prison.

Why a castle?

So what to change?

In summary

Colditz Castle

An impregnable fortress. An inescapable prison.

Why a castle?

So what to change?

In summary

Colditz Castle

An impregnable fortress. An inescapable prison.

Why a castle?

So what to change?

In summary

Colditz Castle

An impregnable fortress. An inescapable prison.

Why a castle?

So what to change?

In summary

Resist battering rams on the gate

Resist Balistas, catapults and trebuchets.

Resist Starvation by means of cutting out supply lines.

Resist ladders and siege towers.

Resist tunnels (bonus)

Resist battering rams on the gate

Resist Balistas, catapults and trebuchets.

Resist Starvation by means of cutting out supply lines.

Resist ladders and siege towers.

Resist tunnels (bonus)

Resist battering rams on the gate

Resist Balistas, catapults and trebuchets.

Resist Starvation by means of cutting out supply lines.

Resist ladders and siege towers.

Resist tunnels (bonus)

Resist battering rams on the gate

Resist Balistas, catapults and trebuchets.

Resist Starvation by means of cutting out supply lines.

Resist ladders and siege towers.

Resist tunnels (bonus)

Sign up or log in

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