How to design a fortress in a mountain to keep it hidden, but still easily defensible?

Question

How could a fortress that could hold around 5000 people with the technology available in medieval Western Europe (1200-1300) be built into a mountain? The fortress would be designed so it remains entirely hidden from anyone casually walking by the mountain, and preferably even those walking on the mountain, yet would still be easy to defend if discovered?

Assume that food and water are not major problems, an underground stream provides water and fish, and there are large storerooms, although ways to acquire additional food would help improve the quality of an answer.

Answer 1

Consider the real world underground city of Derinkuyu in Turkey

They stumbled on a simple solution, a giant stone disc in a groove for a door. Easy to move and brace from the wide interior room but impossible to move from the narrow entrance passage. The narrow bendy entrance made it nearly impossible to break through from the outside without spending weeks or months at it. (literally, hammer and chisel in which time those inside could reinforce the entrance.) the entrance tunnel is so narrow you are not getting more than one attackers at a time. Add a few well-placed holes to drop boiling oil or poke a spear through and it could be defended for as long as the defenders food held out. Note the interior of the city is actually quite spacious.

Hiding the entrances are easy because they are so small, about the size of a normal house door. The city had several such entrances. see the beginning Mormacil's answer for a great idea about using line of sight angles to help hide them, but because of their size even a well placed bush or boulder can hide them from sight.

Answer 2

From walking by casually? Easy. If you assume it needs to remain hidden from the valley below, you just need to place any entrances, windows, etc., behind a curve. I think your biggest issue by far is the population of 5000. That's a lot of people to feed and hydrate. But we're ignoring that problem for the most part I read.

So, before we design the fortress itself: The way to win a siege is by exhausting the supplies of the enemy. Either the besieger gives up because they run out of supplies, food or gold. Or 'they're defeated by a relief force. The besieged loses when their supplies run out and not really in any other way. Except for maybe disease.

So what you want is endless supplies for your defenders. When you don't rely on outside supplies, the siege can go on indefinitely. The only way the enemy can beat you then is tearing down your walls. Being inside a mountain is a clear advantage here. Your walls are hard to tear down if they're literally the mountainside.

For a settlement your size you need multiple entrances; you can't manage just one well-hidden entrance. It would need to be too big. No, you need multiple small ones at multiple levels. Something like this. Gate #2 can easily be reinforced with wooden beams, a metal portcullis, etc. The wooden floor is like a drawbridge. Either that or you simply destroy it if the enemy knocks on Gate #1. The grate will allow you to harass the enemy if they break down Gate #1. Boiling oil, tar, arrows, rocks, etc. Whatever you want and have.

That should allow you to hold the gate for a very long time as long as you have the supplies. For increased security, let your gates go up, inside the mountain. Would make resupplying harder as any cart going into your fortress will need to go up, but will make defense easier.

Light and air

Probably the easiest way to take care of this would be having a hollow mountain or a short canyon of some sorts. With enough time this could be artificially made. Maybe even a steep and dead caldera could work. The majority of your rooms would have a connection to this central shaft. It will bring light down, reducing the need for expensive candles.

This, of course, leaves a rather prime location to attack from. So around the opening, you erect a wall. No gate, just a tall wall you can only access from the inside. Pictured below a round wall. A star shape would likely be superior as it offers no way to scale a wall without exposing your rear (to a volley on pointy things). The stone bridge is my idea of the main access point to the walls. In case they fall you cut the ropes of the bridge. What then remains is this thin strip to walk over. On your end, you either make a shield wall or some wooden screens. Behind them you put pikes. Good luck crossing that. Any ropes thrown or shot to aid can be cut down. And crossing such a thin surface while being stabbed with pikes...

Further down, of course, it makes sense to construct wooden and rope bridges across the level. This would allow your people to get to the other side relatively fast. Doesn't matter if that's easy to defend if you lost both the wall and the bridge above you're already doomed.

Answer 3

I wonder that the other answer did not mention the National Redoubt of the Swiss Army.

It is a well-hidden sytem of massive fortifications inside the mountains, so exactly the thing you are planning to implement. Here are e.g. camouflaged crenels of the Euschels fortification:

Answer 4

Limestone caves

Build your fortress in a region already full of caves and passages. Limestone is absolutely ideal for this, since it naturally forms all kinds of caves and other strange structures. Case in point - the Tsingy de Bemaraha in Madagascar.

You would conceal the entrances to your fortress in existing, natural caves, and yes, that's 'entrances', plural. If you have a single Great Gate, the enemy can locate it and besiege it. Instead, you'd have a main gate, then several smaller passages leading to outlets all over the mountain. That would allow your forces to sally forth during a siege and harass the enemy's rear and supply lines.

Ultimately, the mountain itself is your best defense. It's difficult to campaign in the mountains, and defenders always have an advantage anyway.

Answer 5

A huge crater or depression at the top artificial or not would do the trick, build your fortress in it, site your first contact defenses at the rim, could even have a moat easily enough. Then secondary solid defenses would be your fortress.

Unseeable from the ground, solid defensive site if attackers climbed up.

Assuming the picture is a mountain top, I could imagine a huge fortress instead of the island, perhaps bigger, with terraced agriculture going on all the way around the rims inner slopes and a bit of animal husbandry happening. If it was big enough it could handle 5000 people. An island like Nauru supported 1200 people and it's only 12 miles around and people could only live in a small strip around the edges the whole inland after a few hundred meters was not habitable. Swiss have fish in their mountain lakes. That Island in the middle is probably a volcano plug which is a solid foundation like Castle Rock in Edinburgh.

Answer 6

A castle inside a large cave.

Literally, walls indoors.

So for starters you have the main cave entrance. (tremble at my mighty ms paint skills)

This would be the main entrance only, a good cave fortress is going to have multiple egress points for flexibility.

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Upon walking into the cave you would see this (below): A 25 foot tall stone wall nestled into a narrow point in a large cavern.

• Your cave should allow for no hiding places for attacking forces once they enter the main chamber of the cave. If it naturally does your defenders could carve it out so it is wide open.

• Add a dry moat if you wish, though as you mention there is an underground water source so there is no reason it couldn't also be filled with water...and piranhas.

Additional Design Notes:

• The rest of the cavern beyond the wall could have open spaces or a warren-like cave network, that's really up to you. Just consider enough space for your 5000 people.

• Make the wall tiered so you have multiple platforms from which to rain pointy death upon your enemies.

• It may be worth considering crevices and cracks in the stone ceiling to allow for natural sunlight to get in once in a while. This would also aid with ventilation which is always a concern underground.

Keeping it a secret: This is going to be tough. In an ideal situation...

• Your defenders would also be masons so they could create the stone structures themselves. Otherwise, you have to bring people in to build it and if it has to be a secret they either have to stay or be killed.
• Make it remote. Keeping it off the beaten path will help, ideally, it would exist in a land NOT part of a kingdom that is looking to exert authority over its entire domain...hold outs are annoying to kings.
• Camouflage the entrances, make it harder to find.
• Avoid unnecessary excursions outside the fortress. This is particularly important if the surrounding area is even sparsely populated...people wandering into a mountain and not coming back is sure to arouse suspicion.

Unless there is no one else around it won't stay a secret forever...but if there is no one else around why do you need a secret cave fortress anyway.

Defending it: So, yes, this place is insanely defensible or at least it is against standard medieval warfare. Men charging into the tunnel to assault the gate would be in a cramped space with nowhere to hide. Attacking it in a standard fashion would be completely suicidal and you'd potentially end up blocking access to attack with your army's corpses. Defenders could also always cave in the main entrance if they wanted to...

Weaknesses:

• Fire, or more importantly smoke. As mentioned above a standard attack would be idiotic, and that's even without Spartans defending it. But smoke could potentially be a huge problem. Why attack a cave when you can just control the entrance and roll burning balls of pitch into the cave and asphyxiate the defenders.
• Siege. This place is going to be pretty easy to lay siege to...sure if they can provide food and water for themselves they can last a long time but unless they have access to the outside world they will eventually fall.

Answer 7

Facts

Before we delve into designing the fortress, it seems best to remember a few facts about the period itself:

• recreational climbing/exploration wasn't a thing,
• most people in a medieval era never left their village, the few trappers would follow game, not venture into arid areas for the fun of it,
• trips, whether at sea or in the mountains, were slow-going and dangerous; natural disaster, lack of medical knowledge, wild animals (wolves/bears in mountainous areas) made Nature the greatest enemy, it was expected that travelers may run into trouble and possibly never come back,
• in the absence of science, suspicions and myths ran the gamut.

Picking a location

A remote location is unlikely to ever be visited; which minimizes the risks of discovery. Thus, just picking a remote location off the main travel roads is already a significant step in the right direction.

A seemingly arid location also helps; if there is nothing to gain, no bounty, game or lush fields, then only fools will willingly set up to explore the area.

This can be exacerbated if rumors of dark forces, monsters, spirits, etc... scare people away. If nobody ever comes back, such rumors will form without any active intervention: people wish for an explanation, they'll make one if none is readily available.

Depending on the nature of your hidden people, they may choose to integrate the unlucky wanderers, bribe them or kill them, to ensure their secret remains secret.

Hiding the location

It is best if, on top of being in a remote area, your location is only accessible with difficulty.

I would elect a deeply ensconced valley, cut-off from the world on 3 sides by nigh impassable mountain ranges. Or even the crater of a former volcano.

If there are accessible paths, they can be concealed or rendered impassable, at your leisure.

Remote, inaccessible, and with nothing to earn in sight: why would anyone go there?

Which actually ties in with the fact that you should aim for autarky. Being self-sufficient will mean that your own people have little reasons to wander about. If only a select few (rangers/spies) go out, they can be trained in not leaving obvious trails and concealing their travels. Also, patrols are great to (1) make wanderers go away and (2) warn of incoming threats.

Self-sufficiency is actually easy in this case: you have fresh air, pure water and arable land (at least on the volcano). A few trees would not be amiss for construction material/heating, which if you do not pick too high a spot will be easy.

Note that most of the times your people should live in the valley/crater itself, not the fortress. This makes life more comfortable.

Designing a fortress

The fortress itself will depend on the location. And how hidden you want it to be.

If the location is sufficiently hidden, then the fortress itself can be as blatant as you wish it to be. Obvious locations include:

• the island in the middle of the lake (for a crater),
• the top of the valley, with its back to the mountain and flat terrain in front for shooting practice,
• a cave system.

In any case, an approaching enemy will make a good target for archers, making it costly for an assailant to close in. The lake is maybe the worst case for an assailant, especially if the island is hard to land on (naturally, or artificially).

In the case of the cave, note that you have no need for big entrances, which is great since those are harder to defend anyway. A man-size entrance can be closed off by rolling a stone from the side (good luck to whoever attempt to break through it). Otherwise, hearses/thick doors with egresses to harass the assailant will do, especially if you have several sets of them.

I would recommend putting the food storage inside the fortress itself; this would make it extremely costly and difficult to besiege the fortress, especially as your patrols could harass the enemy, focusing on destroying/spoiling its own food storage in a war of attrition.

Answer 8

You've likely got enough ideas about defensibility..

Feeding the 5000 isn't actually so much of an issue. Crater/Extinct volcano has caldera.

Look up caldera and crater lakes for some inspiration maybe. Oregon's originally named 'crater lake' has 18.7 cubic km of freshwater..So you have a freshwater lake all to yourself, providing your terrain blocks off access to visitors, thankfully you're the world builder.

Now, you don't have to leave that as it is, you've been preparing this bastion for a little while...you could keep part of the interior slopes for growing trees and turn the rest of those slopes into farmland.

If you don't like that idea you can give your mountain fortress access to the sea..doesn't have to be navigable, maybe just tunnelled out angled 'wells' into subterranean saltwater basins fed by the sea..aka a grotto or two for supplementary fishing..

And/or your mountain can have an 'internal access only' ravine open to the sky which (obviously) gets rained on and can be used for agriculture

Recreational mountain climbing wasn't so much of a thing when people had to work to stay alive. You'd likely be pretty safe from discovery even if you put advertising signs out.

Answer 9

Concerning food

Apparently, you need about 1m² to grow the wheat (550 g) for a single loaf of bread. I don’t know about the area requirements of other crops but wheat is pretty energy dense. Those 550 g contain 1925 kcal, so one loaf per defender/inhabitant per day should be enough. For a whole year that’s 1 825 000 loaves for the whole population, using 1003.75 t of wheat and requiring an area of 1.825 km² to grow. So we are talking about whole valleys (e.g. at least 3 km long, 600 m wide) with arable land here, not just a small mountain meadow.

Even if you don’t plant your own crops you can see that it would require thousands of carts to transport the food to the fortress. Of course, if you only have to feed the population during short sieges it would become easier. However, for 3 months of siege, you’d still need 250 t of wheat.

I guess your best bet would be to disguise the fortress (e.g. as a mine) and don’t even try to hide the supply chain.