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The Premise:

I have this story about a medieval general getting besieged in a fortified town. The town is well fortified but the troops defending it, while enough to repel most determined attacks, are not enough to attempt a breakthrough; because of that, after several attempts by the attacking army to take the city, they decided to just starve the defenders out.

The real question

Knowing that his army would probably never be relieved, my general decided to just put the city's populace to work digging a really long tunnel to the outside world. I assume that the tunnel would advance by about 10 meters a day and the city would have enough provisions to last about 1 year; that would mean you get 3.5 km of tunnel, far enough to get out in a nearby wood near the city and evacuate or smuggle in supplies without the attackers noticing anything. It sounds feasible, but I haven't heard anyone doing it in our history, so my question is, why not? Did this ever occur in history? I mean I would do it even preventively, because yes, even if discovered, I can just collapse the tunnel or just easily defend it...

Edit: The town has enough wood to make all the supports and beams needed for the tunnel and space enough to accommodate all the dug-up soil and rocks.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Jan 30 at 18:04
  • $\begingroup$ Have you considered a specific destination for your tunnel? Perhaps your town is near a sea. The harbor/town by the water is controlled by the enemy, but the sea itself by allies. Reach a remote cliff face over the sea, and you can slowly move boats in and out at night/low tide. There may even be an established "smuggler's cave" there, and you are just trying to connect with the smuggler's tunnels. It helps to solve the "control of the other end" issue. $\endgroup$ – DWKraus Jan 30 at 18:45
  • $\begingroup$ Are any of the defenders Foxes? Fantastic ones perhaps? $\endgroup$ – Studoku Jan 30 at 22:20
  • $\begingroup$ This was pretty common except that the tunnels were often built at the same time as the castle, not as an afterthought. $\endgroup$ – chasly - supports Monica Mar 6 at 22:30

14 Answers 14

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The Sarajevo Tunnel

While not medieval, the Sarajevo Tunnel did exactly what you're talking about. Constructed in 1993, it was constructed in order to move people and supplies from the UN controlled airport to the city of Sarajevo while it was besieged by Serbian forces. It was less than 1 km long, and was constructed over a few months. It was dug manually with picks, shovels, and wheelbarrows, so there's no reason it couldn't be done in ancient times (provided they have the technical know how of supporting the tunnel).

Additionally, narrow 1/2 km to 2 km escape tunnels were also often constructed in castles and other structures like Muchalls Castle, and Bishops Palace at Exeter, but isn't typically a feature of an entire city.

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    $\begingroup$ The Sarajevo example sounds like both end of its were controlled by the defenders. That's very different from a situation where only one end is under your control and the other is in the wilderness. $\endgroup$ – Cadence Jan 27 at 19:58
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    $\begingroup$ @Cadence You're right, I never thought of that. Your point completely invalidates my answer, I'm ashamed I even posted such a bad example. $\endgroup$ – Mathaddict Jan 27 at 23:14
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    $\begingroup$ Well. It still works if the defenders somehow do control both sides, such as a tunnel through to an entry point still controlled by the defenders' allies (you could substitute the UN for an allied third party that due to politics can't directly intervene but is too powerful for the sieging army to risk antagonising with an attack) $\endgroup$ – Pingcode Jan 28 at 11:02
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    $\begingroup$ @Mathaddict still valid imho. In sarajevo they just were able to dig from both sides (about halfing the time required). An actual constraint however: "170 cubic meters of wood and 45 tons of steel were used". Which is a BIG number in a medieval city under siege. Also: only half of the tunnel length was actual tunnel, the rest merely a covered trench (aka faster to dig) $\endgroup$ – Hobbamok Jan 28 at 11:05
  • $\begingroup$ @Cadence If the enemy isn't besieging the other end... why would you assume it's not controlled by TGG (The Good Guys)? If it's not controlled by TGG... it might as well be another seige point... $\endgroup$ – WernerCD Jan 29 at 7:27
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There is a main problem here: a besieging army doesn't stand the whole day in front of the city walls, waiting or throwing stones. Some of them maintain their equipment, some other scout the surroundings for ensuring safety and some other raid the surrounding for harvesting resources.

You can smuggle a single messenger out with a pinch of luck, but luck doesn't work for large numbers: either letting all the citizens out or letting supplies in will be discovered.

Moreover, having a tunnel scaled up for a large scale operations means a large tunnel. You can't push a half cow through a narrow passage meant for a crawling soldier at a time. This would be a huge weak point in the defenses: if you assign a permanent watch on the tunnel to prevent surprises, to avoid that one night you will find yourself conquered, those are people you are taking away from the walls.

And don't forget that a tunnel that can be easily collapsed it's a nightmare to be maintained. And a collapsed tunnel will bring damage to what's built upon it, including your walls.

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  • $\begingroup$ Thank you for your answer, but most of what you said in it is easily fixable. $\endgroup$ – TobyB Jan 27 at 11:18
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    $\begingroup$ .... a half cow? $\endgroup$ – Azor Ahai -him- Jan 27 at 21:16
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    $\begingroup$ I think this answer gets the closest to the crux of the matter: It takes a lot of supplies to keep a city alive. The volume of trade that would normally go through multiple city gates now has to go through a single tunnel -and- stay hidden. The best tunnel in the world wouldn't help. $\endgroup$ – aherocalledFrog Jan 27 at 22:11
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    $\begingroup$ @AzorAhai-him-, slaughtered animals are usually transported from the slaughterhouse in halves or quarters. Then the butchers does does the rest of work to get out the various cuts. $\endgroup$ – L.Dutch - Reinstate Monica Jan 28 at 11:33
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    $\begingroup$ Whole cows would be transported much more effectively by trebuchet. $\endgroup$ – Brian Drummond Jan 30 at 13:08
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Is the tunnel to be established on soil or rock?

If soil, how are they going to get timber to line and support the tunnel to prevent cave ins?

If rock, they won't advance 10 meters each day with just picks and shovels. Modern mines, using mechanized electro-hydraulic drills, mechanized underground loaders, underground trucks and explosives can advance tunnels between 10 and 15 meters each day, working 24 hours a day.

What size tunnel (height and width) would they be digging? If 2 meters high by 1 meter wide, for 3500 meters that is 7000 cubic meters of muck that must be removed and disposed of. Using a swell factor of 25% that would produce a conical pile of loose uncompacted muck 20 meters high and 40 meters in diameter. Will the fortress have room inside it to accommodate such a pile?

Then there is the issue of ventilation. How will a 3500 meter long tunnel be ventilated in medieval times. Modern tunneling uses ventilation ducting and fans. Ventilation ducting in mining and tunneling was introduced in the mid 20th century.


Edit

De re Metallica, first published in 1556 might give you some ideas about the technology that might have been used. The English version was first published in 1912, after having been translated by Herbert Hoover, 31st president of the USA (1929-1933), and his wife Lou Henry Hoover.

You can see some pictures from the book, it's 638 page long.


Edit 2

Some long dormant brain cells have just been revived. Prior to the use of drilling and blasting to break rock, fire quenching was used. The rock face was heated by a large fire and cracking and spalling of the rock was induced by dousing the hot rock face will water. Having such fires was a major ventilation issue.

If the rock was limestone, the rock could be softened or partially dissolved using acid. Vinegar was sometimes used, but being a weak acid it was a slow process.

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  • $\begingroup$ The ventilation Factor might be a problem... I did not think of that... $\endgroup$ – TobyB Jan 27 at 11:28
  • $\begingroup$ @Fred You beat me to it. +1 $\endgroup$ – mwarren Jan 27 at 11:29
  • $\begingroup$ Ventilation will be a BIG problem. Normally a tunnel digger would solve this by simply poking holes in the ceiling every once in a while. But that would reveal the tunnel, so not available here. $\endgroup$ – PcMan Jan 27 at 17:19
  • $\begingroup$ Oah! People in the Middle ages were a lot smarter than I thought! Also the ventilation problem could also be solved by some sort of an improvized pipe system with bellows at one end. I mean people must have known that ventilation would be a problem even back then.I mean you can feel asphixiating and desire fresh air. $\endgroup$ – TobyB Jan 27 at 19:33
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    $\begingroup$ Getting bellows to force air 3500 m would be a very large undertaking. $\endgroup$ – Fred Jan 27 at 22:57
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Additional to the other answers, there are some more factors:

  • Scouts in the siege army are everywhere in 10km, to secure the area and also to find food. Starving was a problem on both sides. Your tunnel will have to exit in a secured place outside that area.

  • Spies will tell your enemy what's happening. They were usually in place long before the army was even planning the war. Like today.

  • Fortresses were often built on rock, to prevent tunnelling in the first place. Tunneling is a standard tool for the army laying siege, so the builders tried to prevent it.

  • Fortresses also were often on places difficult to reach, high up for example. This makes your tunnel a lot longer and more difficult to build.

  • Many sieges were laid not at the walls of the fortress but kilometres away - on the streets going in and out of the place. You never see the enemy but also you will see no food coming in anymore, until you give up. On the other hand, this makes building a tunnel or smuggling route easier, if it just has to avoid the road block...

All of those points are not show stoppers of course. Rather they are nice story devices for your book or game. Try read the Simplicissimus. It was written around 1650 and contains memories and stories of the German 30 years war. It's fun to read and gives you some impressions about war in those times, also about sieges in those times. The Author wrote the book after the end of the war, and was 30 years old at the time. So he knew nothing but this war in his life.

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    $\begingroup$ The tunneling by besiegers as standard is important. Even before gunpowder, that was the way to collapse block-built walls. That's why rock is maybe preferred, BUT a moat (not even water-filled!) served same purpose (but here would ruin tunneling-out plans). $\endgroup$ – user3445853 Jan 28 at 12:03
  • $\begingroup$ Being born at the end of a war means you know a surprising amountmostly, but not exclusively, , second hand. In some ways, it gives you a broader view because you rely on the stories of more than one person instead of just your own experiences. And the memories are still very fresh. $\endgroup$ – Mad Physicist Jan 29 at 8:15
  • $\begingroup$ He was born 4 years after the war started. He published 20 years after the end. Historians are quite sure he copied a lot from other authors (Copyright wasn't a thing) and invented large parts of the storyline. But there is the story on one hand and there is the setting on the other hand. And the setting for his story is, of course, 100% authentic - he knew nothing else. $\endgroup$ – Anderas Jan 29 at 14:08
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What you are asking about does exist. Derinkuyu underground city. Large enough for 20k people. Connected to some other cities by many kilometers of tunnels. Something like 200 cities in the region. Large portions built: 800-1100 CE. Used to hide from Byzantines, Mongols, and others.

Big caveat though. This group of cities are all in a particular geological formation. This is important to note. The practicality and logistics of tunneling very wildly depending on rock and soil types.

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In many cases sieges were only extended (by the besieged) till an army could be send to chase the besieging army away or fight them from both sides. (No links or proof, I have read it somewhere, do not remember where.)

In that case, a siege lasting longer than a year to be longer extended is unlikely. If an army can not get there within a year, it will not get there at all.

And your plan of a tunnel underneath a besieging army requires a safe spot beyond that army for your city people to get resources, while these people are clearly not able or willing to fight the besiegers. Rather unlikely.

And the ground should be suitable for tunnels, which is a lot less likely than most people think.

So to make your tunnel successful, it should lead to people who are willing to help but for (explained by you) reasons not able to fight for the besieged.
The tunnel should improve the situation inside the siege lines by enough that the besieged can fight more successful, but I think you should still aim at an army coming in to end the siege.

On both sides the biggest killer was usually disease. And depending on where in the world, winter was the worst period for holding a siege, but it did also help a bit by freezing water that might be used in the defense. So if you intent a round the year siege, consider whether winter will be a problem to either side, and if so, how.

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Why not? Because it's impossible to hide logistics on that scale.


A medieval city is, what, 20,000 people?

60,000 square meals a day. 2,500 meals an hour through a tunnel. A wagon can carry 500 kilos (rough numbers). Let's say 1 kilo is one meal. Oxen travel at 2mph. Horses a bit faster. So let's split the difference and say it takes your wagons 1 hour to get through the tunnel.

So that's 5 wagons per hour, every hour, going in. And another 5 coming out the other way. Even if the enemy don't spot the tunnel entrance, they're definitely going to spot the near-constant stream of wagons going into/out of that stretch of woods from the nearby settlements.

Assume it's a 2-day round trip. That means you're going to need 240 wagons (and hence at least 240 drivers) on the road at any given time. Plus the thousands of people in those settlements either supplying them, or who know them, or who can see what's going on (this is not an operation you can hide).

Are you really saying not a single one of those thousands of people is going to inform on the whole operation for a big reward?

The enemy's scouting parties aren't going to constantly run into these wagons, out in the countryside, laden with massive provisions, heading towards the besieged city but not there to resupply their army, and just let them be on their way without investigating further?


You can change the numbers around to suit your narrative, but I'd say the reason it's never been done for a city is because the logistics are just too big to hide. And trivially easy to stop once they've been discovered.

The only time I can think of it being done is Carthage. And they did it with ships.

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    $\begingroup$ Nobody is going to eat three 1kg meal every day. 200g per meal is reasonable enough as an average and two meals per day would be sufficient, even though people may be sligthly hungry it won't be that bad. In the end that makes 16 wagons per day, with a two days-round trip it leaves you with 32 wagons on the road. It might still be noticeable and the answer might still be relevant, but the numbers were a tad too big. $\endgroup$ – Echox Jan 28 at 11:20
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    $\begingroup$ @Echox Fermi estimation will do that to you ^^ $\endgroup$ – Kaz Jan 28 at 11:54
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    $\begingroup$ Medieval cities were much smaller! 5K--10K except for a handful. E.g. Antwerp had about 5K when they started building their (123m tall) cathedral and were a serious regional presence; Durbuy had a few hundred when given city rights (look at its geometry, you can't fit many more). You can choose whatever size fits the narrative, basically, so maybe smaller is better (which would suit smaller besieging force, and hence smaller logistical headaches). The population is anyway proportional to both labour available and food needed. $\endgroup$ – user3445853 Jan 28 at 12:00
  • $\begingroup$ I read the OP as tunnelling from inside the city walls to a handy wood outside of them $\endgroup$ – CSM Jan 28 at 13:30
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    $\begingroup$ The logic stands though. Even if the rations are far less than 3 kilos a day (say 1 kilo dry mass) you still have the logistical problem of getting the carts and wagons carrying the food to the vicinity of the tunnel entrance outside the city. Unless the enemy commander and his officers are completely incompetent they will be heavily screening all approaches to the city, not to mention daily foraging parties out to collect wood etc for their camp. You simply wouldn't have convoys of wagons rolling up to within 3+ K of the city NOT being noticed. You'd be lucky to get 1 days rations in. $\endgroup$ – Mon Jan 30 at 0:26
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You might possibly do this, but I see three major problems.

The first is where do you put all the stuff you dig out. I will assume your tunnel is 2m X 2m in cross section and 3.5km long. That is 2 X 2 X 3500, or 14000 cubic metres of stuff which you have to store in your town.

The second is, what are you digging through. If it is soil then you will need wooden supports or the tunnel will collapse. Where do you get all that wood from? If it is stone then I doubt you can dig 10m a day.

The third is air, 3.5km of tunnel is going to require a lot of pumped air as one end is sealed until you break out.

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    $\begingroup$ The ventilation is indeed a problem, the throwing ou the stuff isn't. $\endgroup$ – TobyB Jan 27 at 11:30
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A partial yes. Yes you could dig the tunnel.

However, as most of the other answers state, it will not be useful for continuous resupply.

But you question includes "evacuate or smuggle in supplies".

The second will not work for an ongoing basis, but the first is a possibility.

You could use it to get people outside the walls, whether strike teams, evacuating non-combatants, or a full evacuation. The problem is that you are now (only) 3.5 km outside of the town, which has an army around it.

Where are your evacuated people going? They are on foot, and their only supplies are what they can carry. Enemy scouts and troops (likely mounted) are going to catch up very fast.

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You need some help from the topography. Assume the following the situation:

The fortress is built on a mostly rocky face of the mountain as most fortresses. The mountain stretches into the country and has many villages around it. The region fortress built is very difficult to scale. Attackers are more interested in the fortress than the land surrounding it.

Your people would start digging from a soft stop eventually reaching rocky section. After very slowly digging, they reach to a cavern system which is normally inaccessible. This doesn't have to be too far, say 200m from the fortress itself. This system leads to a path (a natural one) within the mountain. It could be some sort of a collapsed section which is again inaccessible from outside and will not be very visible as it is lower than the mountains surround it. Then they travel some 10 kms on this open path which requires more digging/find the path around caverns to get out. Once they got out of this system they end up quite far from the fortress where the seigers are not currently in.

All in all, digging and finding their way around treacherous mountain paths, building short bridges in some sections to allow carts be carried can take as long as you need. But will be possible. And being in friendly territory they can bring in supplies and evacuate people.

Carts could enter the mountain under the cover of the night. These carts could pose as traders between the villages. Obviously you will move only highest calorie content food as you will need to limit number of caravans.

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    $\begingroup$ Ah yes. The snickers bars of the medieval era. $\endgroup$ – Mad Physicist Jan 29 at 8:25
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    $\begingroup$ Peanuts covered in lard and molasses? Not the worst idea... Though there had been not-so-disgusting food for this exact purpose. Churchkhela comes to mind. But I am sure there are others equally if not more calorie rich. $\endgroup$ – Cem Kalyoncu Jan 29 at 10:19
  • $\begingroup$ Peanut butter and butter. :-) Nice yes, throwing in a freshly rediscovered cavern system. Mines of Moria anybody? If there are natural caverns, they could stretch anywhere, have strange creatures for the adventure, and difficult spots to traverse. Nice! $\endgroup$ – Anderas Jan 29 at 20:12
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If you can tunnel out - the enemy can tunnel in

Mining and counter mining were well established military tactics dating back to ancient times and up on through the medieval period, the renaissance and indeed onward - it was still being used used during Word War 1.

A besieging force would establish siege lines and having determined the enemies fortifications were too strong to break through would proceed to tunnel under the walls. The defenders, aware of the risk would in turn try to 'counter-mine' by digging tunnels of their own under the walls along likely lines of approach by the enemy. If close enough opposing teams of tunnelers would be able to hear and feel the other side digging. The defenders would then turn the angle of their tunnel to intercept the approaching besiegers and then try to breach and capture it. If successful they would drive the enemy back as far as they could before destroying the supports or otherwise causing a collapse. Fighting in dark with picks shovels daggers and anything else small enough to take with them.

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Something related in the real world is smuggling tunnels. The longest smuggling tunnel discovered so far between Mexico and the US is 4,309 feet, and that article links to several more long smuggling tunnels.

Usually not a lot of information is available on how they were constructed (for some reason, the people who built them aren't talking) but you can get some ideas about how the entrances/exits were hidden, what they look like on the inside, etc.

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  • $\begingroup$ They aren't talking because the owners ventilated the builders instead of the tunnels, in some cases at least. $\endgroup$ – Mad Physicist Jan 29 at 8:21
  • $\begingroup$ Also, these tunnels are build with access to pretty decent technology as far as I understand (mainly based on American TV shows, but still). $\endgroup$ – Mad Physicist Jan 29 at 8:23
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I am surprised nobody mentioned tunnel warfare as a reason why it cannot be kept a secret.

As mentioned by another answer, mining through solid rock was practically impossible at a proper rate. So building a tunnel would only work if the castle was built on soil, however in this case the enemy also would use sappers to dig tunnels of their own to bring down the walls while the defenders often tried to countermine these tunnels and demolish them early.

So both sides had to "listen" to the enemy, this was achieved via putting beans on a drum underground, if they jumped the enemy was mining a tunnel. This would give away the defenders.

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Let's suppose for a while that the tunnel ends in a place that can be controlled by defenders, there are still big problems with the actual tunnel building:

  1. Rock - it's not feasible to dig a tunnel in every type of ground. For hard rocks you will need explosives or a big amount of time. Sooner or later tools used for digging will break and you will need new tools. A lot of new tools. With medieval technology is usually impossible to dig in a straight line.

  2. Soil digged out has to be removed. Pretty soon you will run out of room for that in a city.

  3. You will need construction to hold the tunnel walls and protect your workers from cave-ins. That's a lot material, probably from wood. You will need access to a lot of wood. Still, it's probable that some minor cave-ins will be visible from the surface, it's not something you can entirely avoid.

  4. Air. The flow of air inside a tunnel is not simple. Nowadays we usually use big fans for ventilation. With medieval technology you will need regular openings to the surface. And these openings can be noticed by the attacking army. This is especially important if the only lighting you have is fire.

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