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There is a medieval city that, for multiple reasons, doesn't have any kind of structural defense, but they decided to build a wall now as there is risk of a war in the bordering kingdom, which can make the city a strategic target for an attack.

Now, they want this wall to be a nice stone wall, and I assume that takes a lot of time, but I can't find any historic example of how much time does it take to build a wall of these characteristics (~4 km long, ~10 m height and ~4 m wide).

Assuming they have all the resources available nearby, and they can have a few hundred workers working in this construction at the same time:

  1. How long would it take (approximately) for this wall to be finished?

Bonus questions:

  1. Would this construction visibly impact the environment? (they would need to get the materials from somewhere nearby)
  2. If they are attacked exactly one month after they start building the wall, in which approximate state would the wall be? (would it be partially defensible or not at all?)

Edit: I should have clarified: I'm asking specifically about a stone wall. I know that there are better short-term solutions (in fact this city is already building a palisade), but they want a stone wall for the future, and that's what I'm asking about. So the answers that just point out possible alternatives are not really helpful, as I'm not looking for alternatives, the question is fairly straight-forward.

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    $\begingroup$ This is comparable to the still existing city walls of Avignon which were 15 years in the making, starting in 1355. I can imagine that depending on urgency, money, available workers and so on, there will be a lot of variance $\endgroup$ – Raditz_35 Aug 13 '18 at 14:48
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    $\begingroup$ Question 3 depends entirely the answer to question 1 and on the method they use to construct the wall and is therefore premature. I suggest you edit it out. $\endgroup$ – Jan Doggen Aug 13 '18 at 14:49
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    $\begingroup$ Welcome to Worldbuilding, Julian L! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox (both of which require 5 rep to post on) useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! $\endgroup$ – Gryphon - Reinstate Monica Aug 13 '18 at 14:51
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    $\begingroup$ Question 2 can almost be considered a separate question. It's easily solvable: calculate the wall volume and compare to what e.g. a stone quarry produces over its lifetime. $\endgroup$ – Jan Doggen Aug 13 '18 at 14:51
  • $\begingroup$ @Raditz_35 thanks, I was looking for a real-life example but I couldn't find exact times, that one will be useful. $\endgroup$ – Julian L Aug 13 '18 at 14:58
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Given the parameters of the questions, the answer is: Not any time soon, if ever, not realistic.

You refined the question to point out that you are not interested in faster fortification alternatives, and a palisade is in place already anyway. What is wanted now is a "nice stone wall" 10x4m, and 4km long. There's a few hundred workers available. I will assume, in your favor, that a few hundred means 1,000.

A stone wall of these dimensions weights approx 440,000 tons. Even assuming it's only a rough (not-nice) stone wall or a "nice" stone wall exterior filled with rubble, the sheer weight is totally no-go given a limited time frame and medieval technology.
A thousand people would have to move around 440 tons per capita (with medieval technology, that is mostly their hands, animals, carts, and treadmills). 440 tons is an awful lot of weight to move around for one person, unless the distance is negligible. If you assume 2 years of time to do the job, that's over 600kg per day to be moved from quarry to worksite, and lifted up, 5m on the average for a 10m high wall. Not counting anything else, that alone is very harsh already.

(Fun fact: I've actually done that haul work once, 25 years ago, so I kinda know what I'm talking abut. 125 packs of tiles ~20kg each, 3rd floor, 3 young, healthy, muscular males working = 40+ trips per head. Takes all day. The first three trips are easy going, then it gets more and more exerting. After 20 trips, you wish you were dead. A week later, you still can't walk without pain. Now imagine doing that for 2 years straight, no holidays, and as someone in typically-medieval health condition.)

Also consider that a "nice" stone wall with more or less regular blocks will need masons cutting stone, which both increases the amount of material needed, and takes time. With medieval techniques, a skilled mason can cut 2-3, if pressed hard maybe 5 shoebox-sized blocks per day.
If one is very optimistic and assumes all your 1,000 workers are skilled masons, all of them working at super killer top speed, and assuming a rubble-filled wall with only a nice exterior, that's something like 200+ blocks per meter needed, or upwards of 800,000 blocks for a 4km wall.
That's two years of work just for the stonecutting, assuming the materials just magically appear in-place when they need them, and blocks are magically placed on the wall and such. No cranes needed, nor people to operate the treadmills, nothing of that kind, all that is assumed to just happen by magic. Realistically, you can at least double the number of workers needed (try and work in a treadmill for 8 hours per day!).

If you assume your nice stone wall being more massive, not just rubble-filled exterior walls, then multiply that figure by 15.

If you assume boulders have to be hauled from a stone quarry some 20-30km away (not unrealistic), add another thousand people only to do the transport. And of course, you need someone who works in the quarry. Plus, all those people need to be fed, and protected from robbers / looters.

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    $\begingroup$ "Rubble filled" exterior walls can be VERY strong. Take a gander at the history of Corfe Castle in England, which had exterior walls of this type. The castle was sieged as late as the English Civil war, and fell only due to a traitor within the walls allowing the attackers in. After the war, and with full access to the castle, the government ordered it destroyed. The explosives dug in under the walls were sufficient to displace half the gateway by over 6ft for example, but the majority of the walls still stand, despite being breached. $\endgroup$ – Baldrickk Aug 14 '18 at 13:32
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    $\begingroup$ Except we know walls of that scale were built, and in surprisingly short periods. In fact, a quick survey shows that 10m x 4m isn't far off from typical dimensions for walls. $\endgroup$ – Rob Crawford Aug 14 '18 at 14:34
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    $\begingroup$ @RobCrawford: Yeah. Only just, not with "few hundred workers" unless you assume 20th/21st century tech. Make that 25,000 workers and I'm with you. $\endgroup$ – Damon Aug 14 '18 at 16:34
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    $\begingroup$ I don't think "impossible" is applicable even with a small work force. And if it's a crash project, a larger workforce is possible. The Constantine and Theodosian walls of Constantinople were destroyed by earthquakes in 437 and 447, then were rebuilt in 60 days. Sure, they already had the materials there, but they still had to lift and secure the bricks and stones in place. $\endgroup$ – Rob Crawford Aug 14 '18 at 16:52
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    $\begingroup$ @RobCrawford The Theodosian Walls were repaired in 60 days by, according to legend, 16,000 labourers, and restoring a wall is way easier (the foundation & materials would still be there after the earthquake) than building one from scratch. Damon is correct that a work force of only "few hundred" is way too small by at least an order of magnitude for this kind of project, if you look to history for examples. $\endgroup$ – Semaphore Aug 15 '18 at 4:28
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  1. Based on the time taken to build a medieval castle, 2 to 20 years.

  2. If you're on stone you're lucky, but a quarry is a fairly big and obvious thing and to build a wall on that scale you're going to be putting a fair hole in the side of a hill

  3. What wall? Oh those plans over there? Yeah basically, still just a drawing I'm afraid. Given suitable manpower you could have a wooden barricade up in time, but you're not going to have much in the way of stone construction in a month. Brute manpower isn't your problem, it's the skilled labour required to quarry cut and dress the stone, build and move scaffolding, keep the tools in good order etc.

Luckily for you, your medieval city would already have a wall, you don't need to build one in a hurry. A city of that period without a wall would be the anomaly. A good wall is a thing of generations, not a rushed new-build.

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    $\begingroup$ Towns could end up without walls: they often outgrew them if things didn't look too dangerous, or even broke the walls down for valuable building materials. And in large empires like Rome, interior towns didn't need walls. If the border shifted quickly or (more likely in my opinion) there was a civil war or succession crisis, a town could end up in trouble before they have time to react. $\endgroup$ – Cadence Aug 13 '18 at 16:59
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    $\begingroup$ @Cadence, I've corrected that to city. The fortifications being a fundamental part of their being from the earliest point as the original British cities were all fortified Roman outposts. (This is a border city according to the question, it really should be fortified.) $\endgroup$ – Separatrix Aug 13 '18 at 17:21
  • $\begingroup$ @Cadence Or cities would simply build another wall around the outgrown city. This would result in an inner wall and outer wall. The city I live in has such walls and there is about 150 years between them. $\endgroup$ – Mixxiphoid Aug 14 '18 at 6:13
  • $\begingroup$ @Cadence The inhabitants of suburbs and villages around a castle used to flee within its walls (complete with their livestock and other key belongings if they manage to) in the case of a large-scale attack. That was a part of the castle owner's protection deal. $\endgroup$ – ivan_pozdeev Aug 14 '18 at 7:48
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    $\begingroup$ Not sure why we're talking about "British cities". This is worldbuilding.stackexchange, where people ask about humans with wings. The quester asker has already stated that the city does not have preexisting defenses "for multiple reasons". One of those reasons could be space aliens for all we know. $\endgroup$ – Pink Sweetener Aug 14 '18 at 11:56
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The Aurelian Wall in Rome is a good example, 19 km (12 mi) long, 3.5 m (11 ft) thick and 8 m (26 ft) high. It took four years, so your wall could be constructed in 1 year, since is 1/4 the length.

In one month you probably have only the outline, the first door and the surrounding wall and a wooden pallisade.

And, yes, it would impact the environment. If you are using "hundred of workers", you are bringing people from outside to build (I was born in a city whose Medieval wall was 3 km long and it had 6,500 people), and they will probably bring their families (masons were "free workers" for this reason, they were one of the few guilds whose members traveled). By, probably, increasing the population of the city a 10% for a year, you will have very busy roads, with materials (they will probably destroy a nearby mountain to convert it in the wall) and with food for the new workers.

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    $\begingroup$ Keep in mind that Roman tech was far above the Dark Period regarding architechture. $\endgroup$ – T. Sar - Reinstate Monica Aug 13 '18 at 20:37
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    $\begingroup$ Time to build does not necessarily scale with length. If you have people working side-by-side along the length of the wall, time to build is independent of length... $\endgroup$ – DJohnM Aug 13 '18 at 22:36
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    $\begingroup$ @DJohnM Yes, but the Romans used more than "hundreds of people", so it compensates. $\endgroup$ – Alberto Yagos Aug 14 '18 at 8:21
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    $\begingroup$ I'm intrigued by this answer, it seems that the Romans built a lot faster than medieval people, were they really that advanced? or is it more a matter of number of people working on the same thing? $\endgroup$ – Julian L Aug 14 '18 at 8:37
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    $\begingroup$ Basically: yes, and yes. They were significantly more advanced, and they had not only a lot more people but the organization and logistics capable of deploying a lot more people usefully. $\endgroup$ – Joe Aug 14 '18 at 16:50
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Under the circumstances, the town shouldn't be looking to build a whole stone wall, they should start with a palisade: a wall of wooden stakes. These are nowhere near as permanent, but they're fast: Roman legions famously built palisaded encampments every single day while on march, carrying the stakes with them. They're also very amenable to using a lot of unskilled labor, whereas stone walls need highly-trained masons to supervise who may not be available on short notice.

Maintenance on such a wall would be fairly labor-intensive (wood decays, and you have a lot of it) so you'd want to replace it eventually with a stone wall that will last for future generations. You could try building the temporary works a little further out and working on the wall behind them in safety, but it's unlikely to be finished in any reasonable amount of time, so your town will probably have to lump it until then.

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1) They could try digging a dyke. A dyke is a deep broad ditch and an earth wall on the inner side of the ditch made with the earth from the ditch. Hundreds and thousands of dykes were made in Europe during ancient and medieval times, and some were tens of kilometers or miles long.

2) Or they could try building a palisade around the town, like Cadence and JoBo12 suggested, and after it was finished perhaps make a second palisade 4 meters inside or outside of the first one. Then they could build temporary wooden ramps up to the top of the outer palisade and did a wide ditch outside the outer palisade. And the dirt from the ditch could be carried up the wooden ramps and dumped into the space between the two palisades until that space is filled with tamped down earth.

Each of the three phases of the project should take months at least, but when its is done they will have a decent temporary town wall, probably after a year or two. And if the situation still seems dangerous, they could build a stone wall inside, or outside, or on the same line as, the double palisade which they could tear down as they build the stone city wall. They might build the new stone wall very slowly if they think that the double pallisde will be sufficient for a decade or two.

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Your city is in the same situation like the Netherlands when it seceded from Spain. In this case the Dutch used for their fortifications mostly earth and water moats.

See a picture of Bourtange: enter image description here
Wikimedia Commons, public domain

The advantage of building the fortresses this way was that it was fast because all materials were already there...the disadvantage was that it needed constant supervision and much maintenance. Simon Stevin was a polymath which wrote De Stercktenbouwing (The construction of fortifications).

Even then, building time was reduced from several years to at least half a year. You cannot expect to have a sustainable fortification ready.

Caesar was able to build an ersatz fortress in the Gallic Wars around Alesia in only six weeks, but this was only possible because all legionnaires were building on the project and were experienced builders (Fortified camps were a necessity for Roman legions).

enter image description here
Wikimedia Commons, CC BY-SA 3.0, 2006-03-11

So no way you can build anything holdable in only a month.

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    $\begingroup$ Note that star forts weren't a thing until firearms were introduced to European warfare. $\endgroup$ – Philipp Aug 14 '18 at 12:46
  • $\begingroup$ Roman legions built a ditch-and-palisade fort, and tore down the previous one, every day while on the march through hostile territory. $\endgroup$ – Rob Crawford Aug 14 '18 at 14:20
  • $\begingroup$ as @Philipp says star forts came after the medieval period had ended and had fairly significant differences in their construction utilizing very thick, low slanted walls to reduce impact from cannon and as firearms had much less of a projectile arc there was less incentive for higher walls $\endgroup$ – BKlassen Aug 14 '18 at 22:55
  • $\begingroup$ @BKlassen While polygon forts came into existence when cannons and firearms became available, those had no impact on the idea of a star fort itself, namely as said in the top answer to allow flanking fire and avoid blind areas where attackers can rally under cover. This is a decisive advantage which is independent of the availability of firearms. So building star forts with higher and thinner walls is a no-brainer. $\endgroup$ – Thorsten S. Aug 15 '18 at 21:10
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One way to build a stone wall is to start with an existing row of stone town-houses and fill them with rubble.

An archaeologist can see that something like that happened in Carthage when a new city wall was built sometime around the fall of the Roman empire ... cutting through an existing suburb.

Existing houses and so on outside the new wall may supply the rubble.

Also it's extremely normal to rob existing buildings for their building stone. So if you have any houses, amphitheatres, public buildings, even "paved" roads (with rubble and sand underneath) that you could rob for stone, that would obviate almost all the work described in this answer.

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The Theodosian Walls of Constantiple run 5.6km; the inner wall is 12m tall and 4.5-6m thick. An inscription found in 1993 indicates the inner wall took 9 years to build, including the gates and towers. This was the primary capital of the Roman empire when it was built, and easily the wealthiest city west of Persia (if not in the entire world). Adjusting for the shorter length you want, figure 3-4 years construction -- and stretch that out if the building site isn't as wealthy.

(source Wikipedia)

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Although Caesar didn't live in medieval times, I would recommend you to look up on some of his famous battles. He very often built wooden walls as offensive/defensive tools to win his battles. (notably Alesia and the buildup to his final battle with Pompeii)

Afterwards you should ask yourself why you even want stone walls. If the threat of war would be so imminent you would be way better of with a sturdy build wooden wall as it would be cheaper and faster to assemble and, depending of the surrounding of the city, you would have to clear the trees even if you build a stone wall as they would provide cover for the attacker otherwise.

Exactly how fast you could assemble a wall also highly depends on the workforce and craftsmen that is available to you.

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    $\begingroup$ I think you meant "Pompey", not "Pompeii". $\endgroup$ – Monty Harder Aug 13 '18 at 18:17

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