Architectural feasibility of a tiered circular stone keep

Question

I have created in my world a keep that lies inside a walled courtyard.

Artistically I have designed the keep to rise in three tiers not unlike a wedding cake. (Though the castle is made of stone not batter. Less delicious but decidedly stronger)

Like so:

After some initial searches online I could find 0 examples of a stone structure tiered in this manner. Which leads me to believe it may not be architecturally possible...which kind of makes sense, though I am no expert in castle construction.

My Question: Can this type of stone structure be built with the following requirements in mind, and if so, how?

• The inside of each tier should be as open as possible, meaning people need to be able to live/work/rain pointy death upon their enemies from inside.
• Medieval construction techniques and materials only
• Cost is not a factor, I am not interested in answers of the "It's not cost effective/why would you bother" variety
• The design is a simple aesthetic choice for the city there is no deep meaning behind it.
• The floors are roughly: 60/40/20 feet in diameter (~18/12/6 Meters)
• I am looking for 25 foot tall ceilings on the main floor and 10-15 feet on the second and third floors. (~ 7.5 M and 3 - 4.5 M)

The best answers will:

• Allow for the maximum amount of open space possible on the main floor. The fewer line of sight blocking requirements the better. Ideally you could stand in the middle of the main floor and see the walls in 360 degrees (obviously some sort of supports will be needed)
• The center of the second floor should be open. Meaning there is an atrium so that you can see down from the second floor onto the main floor

Per several requests I have added an image of what I was imagining the inside to look like.

Answer 1

I am sure that vaguely "Medieval" technology could build a structure like you describe out of stone, and/or brick, and/or tiles, and/or concrete, and/or wood, and/or other materials.

You may have to modify some aspects of your design in order to keep other aspects, and thus may have to make some choices.

I suggest that you take a look at plans (if available) for the various buildings I mention, including looking up other sites besides the ones I link to. You may want to find out how thick their walls were or are and how large the interior spaces were or are.

The Pharos at Alexandria (280 BC)was very tall, allegedly 100 meters (328 feet) or 120 to 137 meters (393.7 to 449 feet), rivaling or possibly surpassing the Great Pyramid (145.6 meters or 481 feet), and probably had more interior space compared to the thickness of the walls.

https://en.wikipedia.org/wiki/Lighthouse_of_Alexandria1

https://en.wikipedia.org/wiki/Great_Pyramid_of_Giza2

In Rome the Mausoleum of Augustus (c.30 BC) had several concentric circular walls, higher in the center, to support a conical mound of dirt and growing plants.

https://en.wikipedia.org/wiki/Mausoleum_of_Augustus3

Emperor Claudius (reigned AD 41-54) built a harbor and lighthouse at Ostia, the port of Rome. It has been suggested that for reasons of prestige Claudius must have built his lighthouse taller than the Pharos at Alexandria. Thus it might have been taller than the Great Pyramid.

https://www.ostia-antica.org/portus/c001.htm4

The Colosseum (AD 72-80) in Rome. The Colosseum is sort of an inside out version of what you want, having an arena at ground level in the center and the highest walls on the outside, so one would have to turn it inside out to get the right plan. And of course the Colosseum was designed with many entrances and exits and you wouldn't want that in a "keep" if the "keep" was intended for serious defense purposes.

https://en.wikipedia.org/wiki/Colosseum5

In Rome the Mausoleum of Hadrian, now part of Castel Sant'Angelo (134-139), like the Mausoleum of Augustus, had several concentric circular walls, higher in the center, to support conical mounds of dirt and growing plants.

https://en.wikipedia.org/wiki/Castel_Sant%27Angelo6

The Rotunda in Thessaloniki, Greece (306) has two concentric circular walls, the taller inner one supported by arches and piers.

https://en.wikipedia.org/wiki/Arch_of_Galerius_and_Rotunda7

The Church of Santa Costanza in Rome (4th century) has two concentric circular walls, the inner one higher and supported on columns and arches.

https://en.wikipedia.org/wiki/Santa_Costanza8

The Church of Saint Stephen in Rome (5th century) has several concentric circular walls, the innermost and tallest supported on columns.

https://en.wikipedia.org/wiki/Santo_Stefano_al_Monte_Celio9

The Church of the holy Sepulchre in Bologna, Italy (5th century, rebuilt c. 1000) Has a taller inner twelve sided wall supported by 12 columns, and a lower outer octagonal wall.

https://en.wikipedia.org/wiki/Santo_Stefano,_Bologna10

The legendary Yongning Pagoda in Louyang, China, built in 516 but burned down in 534, was largely made of wood. It was certainly much taller than the proposed structure would be, so there there would be no structural problems building the proposed structure out of wood.

See posts 88, 89 on page 9 here:

http://historum.com/asian-history/46370-why-do-ancient-chinese-architecture-hardly-ever-go-up-9.html1

The Yongning Pagoda was described in Record of the Buddhist Monasteries in Loyang to be 90 Zhang high and 100 Zhang with the spire, or 330 meters (1082.68 feet), but in the commentary of the Waterways Classic "only" 49 Zhang or 163 meters (534.777 feet). Archaeologist Yang Honxun who excavated its foundations believed it was about 147 meters (482.283 feet) tall.

http://english.cntv.cn/program/documentary/20110531/100055.shtml311

https://www.google.com/search?q=Pagoda+of+Yongning+Temple%2C+Luoyang&oq=Pagoda+of+Yongning+Temple%2C+Luoyang&aqs=chrome..69i57.5655j0j7&sourceid=chrome&ie=UTF-84 12

Anyway, the structure asked about, if made of wood, would be much less extreme than the Yongning Pagoda.

The Church of San Vitale in Ravenna, Italy (526-547) has two concentric octagonal walls, the higher inner wall supported on arches and piers.

https://en.wikipedia.org/wiki/Basilica_of_San_Vitale13

The Church of Saints Sergius and Bacchus in istanbul, Turkey (527-536) has a higher central wall supported by arches and piers, surrounded by a lower square wall.

https://en.wikipedia.org/wiki/Little_Hagia_Sophia#/media/File:Interior_of_Sergius_and_Bacchus_Church-6.JPG14

The Dome of the Rock in Jerusalem (c.685/86-691/92) has a inner, higher, circular wall supported on columns and two outer, lower, octagonal walls.

https://en.wikipedia.org/wiki/Dome_of_the_Rock15

The palatine chapel at Aachen, Germany (c.792-805) has two concentric walls, the inner one higher and supported on arches and piers.

https://en.wikipedia.org/wiki/Palatine_Chapel,_Aachen16

The Old Cathedral, Bresica, Italy (c. 11th Century) has two concentric circular walls,the higher inner one supported by arches and piers.

https://en.wikipedia.org/wiki/Old_Cathedral,_Brescia17

The Rotunda of San Lorenzo, Mantua, Italy (late 11th century) has two concentric circular walls, the higher inner one supported by arches and piers.

https://en.wikipedia.org/wiki/Rotonda_di_San_Lorenzo18

The Garisenda and Asinelli Towers in Bologna, Italy (c.1109-1119?), are basically square stone towers, very different from the desired design, but reached heights of 60 and 97.2 meters (196 and 318 feet) with very thick walls but thin enough to have rooms inside even at ground level.

https://en.wikipedia.org/wiki/Two_Towers,_Bologna19

The Church of the Holy Sepulchre, Cambridge, England (c.1130) has two concentric walls in the nave, the inner one higher and supported on arches and piers.

https://en.wikipedia.org/wiki/Church_of_the_Holy_Sepulchre,_Cambridge20

The Temple Church, London, England (1185) has a circular section with a high inner wall supported on six columns surrounded by a lower circular outer wall.

https://en.wikipedia.org/wiki/Temple_Church21

The Liebfrauenkirche, Trier, Germany (13th century), has a complex, basically cross shaped and circular, gothic structure supported by piers or columns.

https://en.wikipedia.org/wiki/Liebfrauenkirche,_Trier22

The central tower of Lincoln Cathedral, Lincoln, England, was raised to a height of 271 feet (83 meters) in 1307-1311, and a wooden spire was added on top of the tower, allegedly reaching a height of 525 feet (160 meters).

https://en.wikipedia.org/wiki/Lincoln_Cathedral23

The Torre del Mangia, Siena, Italy, (1338-1348) is 334.6 feet (102 meters) tall.

https://en.wikipedia.org/wiki/Torre_del_Mangia24

The octagonal north tower of Strassbourg Cathedral, Strasbourg, France was completed in 1439, and is 466 feet (142 meters) tall. It was the tallest building in the world from 1647 to 1874.

https://en.wikipedia.org/wiki/Strasbourg_Cathedral25

The steeple tower of St. Mary's Church, Stralsund, Germany, built after 1495, was 495 feet (151 meters) tall, which made it the tallest structure in the world from 1549 to 1569 and from 1573 to 1647.

https://en.wikipedia.org/wiki/St._Mary%27s_Church,_Stralsund26

The central tower of Beauvais Cathedral, Beauvais, France, completed in 1569, was 502 feet (153 meters) tall making it the tallest structure in the world from 1569 to 1573.

https://en.wikipedia.org/wiki/Beauvais_Cathedral27

A common early type of castle was called a "motte-and-bailey" castle. The bailey was a courtyard with buildings enclosed by a stone or wooden wall. The motte was a conical mound with a flat top. A usually circular keep of stone or wood was built on top of the motte, and the keep often had a central courtyard with a stone or wood tower in the center.

https://en.wikipedia.org/wiki/Motte-and-bailey_castle28

You give the total height of the rooms in your tower as 45 to 55 feet. You don't say if the ceilings should be flat wooden floors or vaulted stone, brick or other masonry, so I don't know how much they add to the height, and there may be parapets going a few feet above the roof. But your tower looks like it will be less than 75 feet in total height.

You say that say that "cost is not a factor". I suggest that you take a look at the castle of Coucy, built by the Lord of Coucy (not king, not duke, not count, but lord).

https://en.wikipedia.org/wiki/Ch%C3%A2teau_de_Coucy29

Also the Great dome of the Florence Cathedral, Florence, Italy (1420-1436), could easily contain your entire proposed tower.

https://en.wikipedia.org/wiki/Florence_Cathedral30

Answer 2

You can't get it all at once.

The problem with that design in stone is you lose a lot of internal space and waste a lot of material.There has to be a wall under each of those external walls that runs all the way to the ground and further, so the lowest ring has to have three wall along the outside to support the floors above, it is a lot of wasted stone and space for no benefit. Building a single wall taller is more structurally sound. All the weight bearing walls need to go all the way down. The lower levels end up with walls so thick the internal space is the same as the highest section. You can do it but it a huge waste and stone buildings are expensive. You can build a taper into a building but nothing that extreme, you do it by having each wall get thinners as it goes up, that is as much as you can do it, how much you can taper the wall itself. It works with cakes because they are solid and uniform.

Another problem you have is your walls, walls on a keep are several feet thick The upper part of the tower only has room for a staircase. Keep in mind the lowest levels will only have a single at most 20ft wide room in it. 60ft minus 4-5 per side per wall, so you lose around 30ft of the inernal space before you consider how to get from floor to floor.

So if you are trying to maximize your internal space this is an awful choice, you don't have useful rooms in this structure you have a single central staircase and maybe enough room for a chair or two. It also lowers the defensibility of the structure, since the lower tiers create a shadow for fire from the upper ones. Real keeps had straight sides for a reason.

To be clear you can build that shape but not with those dimensions if you want any space inside. If you want that profile/shape and size, you need to give up on having much in the way of internal space.

If you are trying to maximize space just a straight (or mostly straight) walled 60ft wide tower is better, and completely realistic.

If you want that shape and internal space you need to build it much bigger, basically the upper most section needs to be wide enough to hold whatever internal space you want on the ground floor.

Answer 3

What you want is a circular ziggurat. Not any kind of ziggurat, but a hollow one with lots of internal space. That's because not all the ones built by the ancient were hollow in all layers.

Anuway, this is Chogha Zanbil. It was finished around 1250 BC:

This is a view from above:

And this is an artistic rendering of it in its full glory:

If the ancient could do it, then the middle agers could too (well... Maybe, depending on where they are and the plagues they are facing). It just takes a little bit of i genuity to make the whole thing rounded.

Answer 4

I’m not an architect but I still have some opinion on it considering some basic static physics. The main problem with any high building and heavy materials on top some of that force will push onto the wall. Now add windows and not to big walls to the mix and your walls won’t hold against the sideways forces.

That the reason buttress were invented.

https://www.thoughtco.com/what-is-a-flying-buttress-4049089

Depending on your internal structure of that building so thing similar might be necessary here too.

Answer 5

Like a cake, you need to support the upper layers. With cakes they use dowels, or other inserts, to support the upper layers.

What you could do would be to use archways and pillars to create the central ring and build it up to the height of the third floor. Around that build another ring up to the height of the second floor, and lastly build the outer wall for the first floor around everything. Put the cross braces from the outer walls to the inner walls, and then the center, lay flooring on top of that.

If you can make a three story building, then build a two story building around it, then build a one story building around that, then this shouldn't be too hard.

Answer 6

Fake it.

If it's purely an aesthetic choice, then it doesn't actually need to be 100% stone--it just needs to look like it. Given that many (perhaps most) medieval keeps were whitewashed anyway, you can build a wooden tower and whitewash it and no one will be able to tell the difference from the outside. And given that the highest tower is, well, high, and set well back from any ground threats by the surrounding walls of the lower levels, there's no real defensive need for it to be made of stone.

So, you start with a 40-foot wide stone tower. The individual floors within the tower are formed from wooden beams supported by sockets and corbels on the interior wall; critically, the uppermost floor has additional support in the form of diagonal bracing beams which will help to transfer the load of the 20-foot wide wooden tower that you are going to build on top of it out to the stone wall. You will also want to put an overhang on the top floor of the middle tower, with (whitewashed) projecting wooden battlements forming the top of that tower, rather than a straight wall; the reason for this is to allow your floor joists to extend past the top of the stone wall, and then rest weight on them to provide counter-torque to further balance the weight of the central tower resting on that floor.

Then you just building 60-foot-diameter lower level wall around that, and hang the roof and any intermediate floor joists between the inner and outer walls.

The 40-foot diameter stone tower is now going to be your primary structural support. It can have arched doorways cut through it periodically, but you just aren't going to get a full 60-foot wide open space anywhere. Instead, you can have rooms in the middle tower that are up to maybe 30 feet across (subtracting for a conservative five-foot wall thickness), with some encroachment on the headroom at the edges on the top one or two interior floors because of the presence of the supporting braces, and then you get a thin annulus around that. And then you need to add stairs, which I would wrap around the interior of the central structural tower, although that further encroaches on the available room for, well, rooms.

I would probably make the lower, outermost wall considerably wider, so that you can actually get some decent space in that annulus, rather than having most of it taken up by just the thickness of the wall itself. Keep in mind, however, that you don't need to set aside room for hallways; medieval interior architecture didn't really use them, preferring to just have rooms leading to rooms--and if you have to go through some irrelevant room to get to the one you actually want, so be it. Having specialized hallways so you can get from one room directly to any other only passing through public space is an unnecessary luxury.

Answer 7

There is no problem with the structure you have in mind. The choir of a medieval cathedral is quite close to half your structure.

So, you will have three rings of pillars, stacked on top of each other. It's actually very close to the way things were built anyway, so just have fun, build it, and enjoy.

Mind you, your pillars will be fairly massive, since engineers at the epoch did not calculate the structures, but went by try and error. Just look at any cathedral of the epoch, and you will get a good idea how open the space on the inside will turn out.

Answer 8

Tolkien's Minas Anor had this design, but on a much larger scale. It had seven levels, each with a curtain wall and gate. The gates were offset from each other to maximize the opportunities for defense. The fortress was built on an outcropping of a mountain range, so each wall was directly supported by bedrock. To quote the first chapter of Book V of The Lord of the Rings:

For the fashion of Minas Tirith was such that it was built on seven levels, each delved into the hill, and about each was set a wall, and in each wall was a gate. But the gates were not set in a line: the Great Gate in the City Wall was at the east point of the circuit, but the next faced half south, and the third half north, and so to and fro upwards; so that the paved way that climbed towards the Citadel turned first this way and then that across the face of the hill. And each time that it passed the line of the Great Gate it went through an arched tunnel, piercing a vast pier of rock whose huge out-thrust bulk divided in two all the circles of the City save the first. For partly in the primeval shaping of the hill, partly by the mighty craft and labour of old, there stood up from the rear of the wide court behind the Gate a towering bastion of stone, its edge sharp as a ship-keel facing east. Up it rose, even to the level of the topmost circle, and there was crowned by a battlement; so that those in the Citadel might, like mariners in a mountainous ship, look from its peak sheer down upon the Gate seven hundred feet below. The entrance to the Citadel also looked eastward, but was delved in the heart of the rock; thence a long lamp-lit slope ran up to the seventh gate. Thus men reached at last the High Court, and the Palace of the Fountain before the feet of the White Tower: tall and shapely, fifty fathoms from its base to the pinnacle, where the banner of the Stewards floated a thousand feet above the plain.

Answer 9

You're building The Colosseum (arches in a circle), using modular construction like Sears' tower, to a height of less than 200 feet just like the Monadnock building, otherwise the walls, columns and archways need to be so thick that there'd be no usable space on the lower floors.

What you'll end up with is what looks like how most artists depict the Hanging Gardens of Babylon (image search), except circular.

Answer 10

You want the Tower of Babel

So, this is sort of make believe, but literally every Early Modern artist pictured the Tower of Babel as a multi-tiered wedding cake, more or less, although the tiers were often more of spiral, I suppose.

That doesn't make it more physically realistic; other answers address that issue. But, you are in good historical company envisioning a tower looking like that.

Pieter Brughel the Elder, 1563

Athanasius Kircher, 1679

Lukas van Valkenborch the Elder, 1594

Answer 11

The answer is that yes, you can do this, and there is a very famous, well-documented real example that is almost exactly what you describe:

The Light House of Alexandria

The wikipedia article describes it pretty well. The catch of course is that it is 1) Expensive, 2) difficult to build, and 3) might be unstable (the famous Lighthouse collapsed during a minor earthquake).