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Assume that the time period takes place at some point before cannons and other later siege equipment (medeival, antiquity, bronze age, etc.) Stone mages can affect materials that follow the geological definition, which are "naturally occurring and coherent aggregates of one or more inorganic minerals".

Important note is that it only affects compounds with a crystalline chemical structure, not amorphous ones such as obsidian and other glasses. It doesn't affect organic minerals, or ones that are currently within a living creature (such as bone). So things such as Clay, Sand, Crystals, Glacier-ice, and other types of rocks ARE able to be moved, carved, and otherwise manipulated, while glasses, metals and other materials ARE NOT. An interesting case is dirt, which is only around 50% sand/ silt/ clay, and the rest of it is air, water, organic material, etc. Dirt can probably be controlled, but less efficiently than regular stone.

Magic is relatively rare in this world, but widespread enough that most organized armies would have multiple stone mages, and it really only takes one or two of them to easily knock over any wall built out of stone. To control stone in this magic system, the mage has to 'resonate' with the stone by playing some type of rhythm associated with it (similar to an instrument, except using their own body, i can explain this further if it helps answering). Different types of stone have slightly different rhythms that the mage needs to play to control it.

I have considered things such as wood, but the issue is that wood is flammable, and easy to throw a large boulder at using stone magic. This leaves basically two solutions that I've thought of:

  • Find some type of material that stone mages can't easily control, is cheap/ easy to source, and can be used to build large structures OR
  • They don't build wall at all. Similar to how castle walls fell out of use after the invention of cannons and mortars

Edit: also it's easier to control rocks that are closer, compared to further away.

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    $\begingroup$ It failed to embed for some reason, but here's the article i tried to link explaining why glaciers are considered a rock: usgs.gov/faqs/…. I'm pretty sure this isn't super relevant to the answer though.... $\endgroup$
    – Foosic17
    Commented Mar 26 at 19:12
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    $\begingroup$ We need more information. If a wooden wall of any strength is vulnerable because a stone mage can throw a large rock at it then we need to know how far they can throw a rock of what size - if they can repeatedly hurl wall-breaking boulders further than a trebuchet can then the material a wall is made of is almost irrelevant and stone mages can destroy anything. Assuming that's not the case, we need to know about the "in a living creature" limit - can a wall be covered in living vines as protection? "Naturally occurring" - does that include baked clay bricks? $\endgroup$ Commented Mar 26 at 21:23
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    $\begingroup$ The information that the mage needs to be touching the rock is vitally important and needs to be edited into the question body. Given that battering rams and sappers (not to mention people with ladders) need to be kept away from walls, this ends up making much less difference to siege techniques. $\endgroup$ Commented Mar 27 at 0:44
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    $\begingroup$ Some crucial information is missing: what is the effective range of the stone mages? Do they need line of sight? And if there is some material they cannot affect, intermixed or overlaid, does it completely block the magic or just makes it slightly more difficult but still possible? For example, bone ground to dust and intermixed with the mortar, or ivy covering the walls, would it completely block the magic or just slightly reduce its effectiveness? $\endgroup$
    – vsz
    Commented Mar 27 at 5:14
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    $\begingroup$ Big pieces of wood aren't nearly as flammable as you might think. You either need a lot of heat, a lot of accelerant, or a lot of oxygenated surface area (i.e. the pile of little twigs that you'd use to start a camp fire). People were using wooden stockades for fortifications until the 20th century. Double wood-plank walls packed with sand or gravel to make them bulletproof are used in fortifications to this day. $\endgroup$
    – g s
    Commented Mar 27 at 7:22

15 Answers 15

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A Moat

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If the moat is large enough, the mages can't get close enough to any walls without going for a swim first.

I'd argue a stone mage can't affect stone if they aren't touching the ground.

This would mean the can't just use a boat to get close enough

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    $\begingroup$ You drop fire on them if they try to use a boat. $\endgroup$
    – Mary
    Commented Mar 26 at 23:25
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    $\begingroup$ It reminds me of a yt video where someone analyses some fortifications in media. He keeps on asking "where is the ditch?" Any ditch (or moat if filled with water) is an incredible effective technology. Armies are unlikely to storm up or down, it tires them, and prevents siege weapons from easily reaching the wall. Your mage is no different, requiring to go up and down a ditch or two while under fire, possibly needing to destroy one or two waist high walls to improve the advance. Water for a moat often isn't available, but ditches can be made everywhere. $\endgroup$
    – Trioxidane
    Commented Mar 27 at 8:57
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    $\begingroup$ Watery moats are no basis of government. But yeah, if you look closely the basis of the whole thing is build from stone. Resonate with it, remove it, sink the whole artificial island into the moat? $\endgroup$
    – Pica
    Commented Mar 27 at 9:29
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    $\begingroup$ Since the stone mages can effect dirt (to a lesser degree), they could start at the edge of the moat, raise the dirt up, and make a walkway. All they need is some cover / protection from arrows, oil, etc. $\endgroup$
    – LukeN
    Commented Mar 27 at 14:40
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    $\begingroup$ @Foosic17 There is possible solutions for water mages too. Moat is still viable. Look up "Pouring oil on troubled waters" newscientist.com/letter/mg18725131-000-oil-on-troubled-waters $\endgroup$
    – Thorne
    Commented Mar 27 at 22:45
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Warding Spell

Magic problems require magic solutions. When there is magic in your world, then you can make up the rules for how that magic works.

So why not add the ability for mages to ward objects against manipulation by other mages? Anyone who built a defensive structure would then get a mage to cast a warding spell on it to protect it against manipulation from rock mages. In order to not make this too overpowered, those warding spells might require regular renewal and/or might be possible to undo with a lot of effort from the rock mages.

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    $\begingroup$ make it so the ward infuses the material with the caster's mana, and enemies must first flush this mana, effectively discharging the ward, before they can put their own mana inside the rock. This satisfies the need to renew the wards and also makes it a war of attrition, where opposing mages fight for control over the stone — with the defenders having an advantage they can pre-load their own mana. $\endgroup$ Commented Mar 27 at 12:47
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    $\begingroup$ The good thing about this answer its that it avoids any unintended consequences you need to consider in your castle designs. Adding any magic to a setting changes a lot of fundamental assumptions about castle design, but if you want to keep a castle a castle, then magic wards not only keep out the geomancers, but can prevent all sorts of magic from rendering castles irrelevant. $\endgroup$
    – Nosajimiki
    Commented Mar 27 at 16:28
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Most historical castles would be fine

Most historical castles were made of wood, not stone. Our modern perception of stone castles being so common is a survivorship bias because the wooden castles have nearly all burned or rotted away by now. Even most of the castles that appeared to be made of stone were actually wood with a thin facing of plaster. On top of this, the typical things that protect a castle still apply like motes, archers, and boiling oil which would all make getting a geomancer close enough to do his job rather difficult.

The biggest threat a geomancer posses to a typical castle wall would be to delaminate the plaster exposing the wood to burning, but there are other available substances that could be used to fireproof wooden walls which a geomancer could not manipulate like resin.

Your real area of concern should be the castle's foundation.

One of the hardest things to protect a castle against is sapping: the practice of digging a tunnel under the walls to collapse them. A geomancer could sap a castle way faster and more easily than a crew of guys with pickaxes and shovels. This means it is not so much the castle you need to apply major redesign work too as it is the land on which it is built.

Use a Pyromancer to glass the landscape

In order to beat a geomancer, you need to first convert the whole landscape into something that a geomancer can not manipulate. Since glass is immune to the geomancer's magic, a pyromancer may be needed to prepare the castle's foundations. He could over the course of months or even years melt layer after layer of piled on sand building up a thick and massive slab of glass on which to build the castle. Even if a geomancer could eventually clear enough of the land under the thick glass foundation to cause a collapse, this would be a huge magical undertaking, on par in difficulty with traditional sapping, meaning that even with geomancers, a siege could drag on for months which would justify the continued use of castles in your setting.

Likewise, you could make your castle walls themselves out of glass, while we think of glass as a weak material, this perception just comes from our experience with the thin glass we use for windows and light bulbs. But, if you were to make a full thickness castle wall out of glass, it would be tougher than most kinds of bricks and stones that were actually used in castle building.

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    $\begingroup$ +1 to intense Fire treatment. Intentionally melting the whole surface of stone castle means it's no longer manipulable by Stone magic. $\endgroup$ Commented Mar 27 at 22:56
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    $\begingroup$ Many historical castles made of stone would also be fine, limestone, chalk, flint which were commonly used are either organic or amorphous. $\endgroup$
    – John
    Commented Mar 29 at 2:25
  • $\begingroup$ AFAIK the typicality of boiling oil is unhistorical. I.e. if people happened to have lots of oil, they used it, and there are some documented incidences of just that, but nobody likely kept lots of (expensive, perishable) oil around for the purpose. Of course maybe in this medieval setting mages extract large quantities of cheap, imperishable fossil fuels and people store them for use in war... $\endgroup$
    – g s
    Commented Mar 30 at 3:36
  • $\begingroup$ @John Things like Limestone may or may not be considered organic depending on the definition you use. In Chemistry, the term "organic" means having a molecular structure of chained carbon covalently bound to each other and other elements; so, things like limestone, chalk, flint would not be safe because they are made of the inorganic remains of organisms. As for amorphous rocks like Opal, Obsidian, etc. these are not common enough in nature to actually make a historical castle out of on thier own. $\endgroup$
    – Nosajimiki
    Commented Apr 7 at 22:44
  • $\begingroup$ @gs It was probably not like in hollywood movies where you poured giant vats of the stuff on invaders, but there are several examples of machicolations stained with various oils. Boiling oil was in many cases preferred over water because it has a higher boiling point. When you pour boiling water from high up, it spreads out and quickly cools making it hard to actually burn someone with, but oil can get twice as hot, and creates less surface area for cooling, plus it sticks better to your victim. $\endgroup$
    – Nosajimiki
    Commented Apr 7 at 23:05
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You interlock the rock- and you let your mages resonate first with it and turn the wall into a series of golems, more entangled then the couples in a kamasutra book.

Then you add a ablative outer armor layer. Almost all magic falls apart at that level. Instant death spell? Haha, i wear a leather armour filled with layers of yeast. Have fun, killing a million fungi. Same concept, but with pebbles or sand behind a thin layer of wood or mixed into a clay plaster.

Finally, some pieces of rock, just to backfire in front. Long, sharp and buried with just some pillar going up in front of the fort. Topple it over and the buried blade slices your lined up army into a nice charcuterie board.

Others can be used via pulley system, to launch catapult equivalents. Hope those stone mages are fast, cause its rock and its coming that way.

Finally, on the top of the walls - the razor-windmill on a rail. Nothing fancy anti-magic, just a good old anti-ladder device, going back and forth. The walls of the fortress used to be higher, but after so many sieges and so many burials, the surrounding grounds got all uppity.

Yes, im fun at D&d parties.

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    $\begingroup$ I can only resonate with the second idea, mixing stone with something else to make it not resonate with the mages. Otherwise you seem to damage or even destroy the wall yourself with fanciful plans that should lay waste to armies, but could easily backfire or triggered harmlessly. Leaving a city with a ruin of a wall. $\endgroup$
    – Trioxidane
    Commented Mar 27 at 9:03
  • $\begingroup$ The idea is to have the mage resonate with a little pebble. Again and again. Never getting a hold of the stone. Medieval walls are a useless concept in any world that has artillery or magic similar advanced. So i ran with it making it a vertical minefield.. $\endgroup$
    – Pica
    Commented Mar 27 at 9:27
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Defeat the stone mages with the power of Rock

(Or any catchy music appropriate to the setting.)

The stone mages are only really a threat during serious assaults, as otherwise the same defenses that stop small groups of men from casually infiltrating the city with their weapons will stop them from getting close.

The stone mages must play a very specific rhythm to attune to the stones.

Consequently, when the assault begins and the defenders rush to the walls, they all sing out a catchy song in the wrong rhythm for the wall materials. Even if the stone mage can sense the rhythm of the stone over the rhythm of the song, and even if the stone can hear the stone-mage's rhythm and respond, the toe-tapping catchiness of the defenders' song will make it almost impossible to concentrate on the specific rhythm of the stone.

Smaller forces rely on stomping, clapping, banging weapons on shields, and belting out popular tavern songs whenever a stone mage assault is suspected. Better-defended fortifications have dedicated pipers and drummers. The best-defended fortifications enjoy the services of a sound mage amplifying a small group of professional musicians who can switch instantly from a slip jig to a slow waltz at the first hint of a tremor.

(An international treaty organization once attempted to ban the use of bagpipes as a stone-mage deterrent because they were simply too horrible, only to learn in horror that the Scottish would still be using the bagpipes even if they lived in a universe without stone mages.)

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Idea 1: Frame Challenge

How do you mages know what the Wall is made of, if they can't see it?

So, I'm originally English - and I remember very well Ivy-Covered old Buildings: Google Search - Admitedly most of these are backed by Stone - but if the Mage cannot see the stone, Are they able to target it?

Alternatively, you could have a stone wall, but covered with a face plate of Metal (specifically an Amorphous Metal...) - so that the wall would appear to be of all Metal construction.

If your Mages required Line-of-sight or at least knowledge of the stone (e.g. they need to see or know which type of stone is used to know which Magic incantations to cast) then obfuscation provides a pretty good defense. However, if they only need to know the Stone - you could have fun where a Clever Mage would know the common types of Stone in use around the areas and would target those first - so a Welsh mage might try Slate first.

Idea 2: Earthen walls

You said Dirt can 'probably' be controlled - so let's think about Terraced Earth Works - although they take a lot of effort to build (much harder than Stone) - they do have some interesting properties when it comes to stopping projectiles (a Semi-soft Dirty will absorb a Cannonball without loosing much structural strength) - Add in some Wooden retaining walls, you have a pretty strong and resilient series of fortifications - The Mages might be able to target some of the particles in the Dirt - but the question is whether or not there is enough concentration of them to be effective - maybe, maybe not.

A Material like Peat Bog for example which is mostly decayed vegetation would be an excellent choice.

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  • $\begingroup$ I think the issue with peat big is it is highly flammable. $\endgroup$
    – DKNguyen
    Commented Mar 26 at 22:52
  • $\begingroup$ metalic glass, aka amorphouos metal, was invented in 1960 and require modern industrial methods. $\endgroup$ Commented Mar 27 at 12:45
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    $\begingroup$ A less flammable alternative to peat is cob. It's made from soil, water, chalk, and straw. It's very strong, fireproof, and contains a lot of organic material that a stone mage can't mess with. $\endgroup$
    – bta
    Commented Mar 27 at 21:36
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Insulation and acoustics

The mages could just resonate with the stone via magic, but I assume there is a physical component. Something that only the mages can hear, like a dog whistle mages can hear when listening for it. The composition and warmth of the stones make the molecules tremble on a particular way, letting the mages know the composition and how to manipulate it.

Acoustics

The first one is acoustics. I am no expert in audio lingo and practices, so don't expect pinpoint accuracy, but it should suffice for the answer. There are two ways we can approach this.

Fist is confusing. Resonant patterns are repeating patterns. If you get two stones you have two patterns. Two repeating patterns of different stone at once might sound like a different rock, or not truly resonate at all. Thus the mages are unable or severely weakened in their ability to manipulate the stones. A sort of composite armour for city walls. Some stones might also be overwhelming in their resonance, so a small line of these inside the walls might be so 'loud' that the stone making up the majority of wall can't be heard.

Secondly think of concert halls. They are build in such a way that they should have little reverb, while able to reach every seat in the house. The sound can be redirected. The acoustics could be used to redirect the resonance away from the outside. Think of stealth fighters using special paint and shape to either trap or redirect radar at oblique angles. Your walls are the first 'stealth' technology, using specific material to hide the resonance.

Insulation

I live in a relatively newly build house. My brother doesn't. I can barely hear my neighbours at any time of the day if it's dead quiet in my house. He can just hear them. The difference? Insulation.

Insulation does more than just keep in heat. It dampens sound. What does this mean? It means we can start with acoustic dampening or changing. The wall can have a thin layer of something that dampens the frequency, making it too low to 'hear'. This insulation can also change the resonance if it does leave the wall. Thus you have defeated the mages, or the mages need to take big risks to get close enough to hear properly.

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Concrete

Your definition for what stone mages can affect starts with "naturally occurring." Concrete is not naturally-occurring. Maybe the stones inside the concrete can be manipulated, but not any of the binding mortar.

Concrete has been around since Roman times, so easily meets your technology criteria. They could not make reinforced concrete as well as we do now if they cannot produce iron, but bronze or even wood rebar could be used and at least slightly effective.

If the mage has to resonate with the stone to affect it, and that resonance has to travel through the ground, simply building traditional stone walls on top of a concrete foundation would prevent manipulation. If the magic requires line-of-sight, cover the stone wall in a concrete shell.

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    $\begingroup$ You can even make concrete using materials of organic origin, like coquina - an aggregate stone made from the compressed remnants of coral and shells. That would further reduce a mage's ability to manipulate it. $\endgroup$
    – bta
    Commented Mar 27 at 21:29
  • $\begingroup$ Concrete is made by growing calcium oxide crystals around sand and stones. As per the OP's definitions, I don't think concrete would be any different than any other aggregate stone. $\endgroup$
    – Nosajimiki
    Commented Mar 27 at 22:24
  • $\begingroup$ @bta Depends on if you go with the chemical or vital force definition of "organic". As per modern chemistry definitions, coquina is an inorganic material, but before the 1800s it would have been considered organic as being a material originating from a living thing. $\endgroup$
    – Nosajimiki
    Commented Mar 27 at 22:45
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Two ideas:

[Stone magic] doesn't affect organic minerals, or ones that are currently within a living creature (such as bone).

(1) Cover the stone with moss. That might qualify as "within a living thing", or at least block line of sight or magical resonation.

Magic is ... widespread enough that most organized armies would have multiple stone mages... To control stone in this magic system, the mage has to 'resonate' with the stone...

(2) Fight magic with magic: Have your own stone mages on the other side of the wall, actively touching and resonating with the wall. They have the advantage of proximity and the chance to spend lots of time preparing, learning to resonate with the particular stone of the wall. Bonus: if they are in a bind, the defending stone mages can use parts of the wall as projectiles also.

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  • $\begingroup$ I like #2. Lots of opportunity for the old guy who knows the rock so well that he can be drunk and asleep and still be able to do <amazing thing> (detect armies, insult someone by forming words on the walls, etc). Or the recruit who just has to sit around and resonate all day, and instead does <stupid thing> (e.g. sleep and resonate with the stairs (instead of the wall) so that he is awake and busy when any inspector comes, etc) Someone could make a 4" Hadrian's Wall to detect large groups passing--except for that one bit where some idiot who doesn't know geology used the wrong rock!! $\endgroup$ Commented Mar 29 at 17:36
  • $\begingroup$ Upvoting for (2)! In our world, it's already typical for physical attacks to happen in parallel with virtual ones, where electronics and computer mages (er, specialists) on both sides struggle to disrupt or defend every communication, control, and targeting system on the battlefield. As you point out, the defenders' stone mages will have the advantages of proximity and preparation, so the attackers may need to back up their own stone mages with conventional wall-breaking units, like catapults and battering rams. $\endgroup$
    – Vectornaut
    Commented Mar 29 at 22:57
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Different types of stone have slightly different rhythms that the mage needs to play to control it.

Build the walls of many types of stone at once, the more mixed the better. That way, to deconstruct the wall, a mage needs to keep switching between different rhytms, at any time controlling only a small fraction of the wall. All the uncontrolled stones get in the way, blocking most of controlled ones from being moved (they can be specifically shaped to magnify this effect).

Deconstruction of such wall may become feasible if one uses a group of mages, each controlling different kind of stone and cooperating closely.

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Amorphous Bricks

Most clay around the world is mostly made of crystalline stuff, but some clays are amorphous. Even crystaline clay is not 100% crystals and you could centrifugue it to separate the crystal and amorphous phases.

Make bricks out of the amorphous stuff and the earthbenders won't be able to bend those.

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One more idea that I haven't seen mentioned here: your stone mages affect each rock individually. That is, if they have a boulder, they can do whatever with it. If they have a pile of pebbles, they can't affect them all at once - they need to focus on a particular pebble, one at a time. So that's how you make your walls - many small pebbles glued with cement. Have fun trying to dismantle it!

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Bone should be a pretty readily available material if your people eat meat. Or they could scavenge bones long abandoned by predators, or whale bones(etc), washed up on beaches.

Here is a house our stone age ancestors built entirely from mammoth bones. Mammoth bone house

https://www.thevintagenews.com/2020/03/23/mammoth-bones-structure/

Antlers or tusks could also be used. Antlers can easily be picked up each year after deer shed them.

The wall could be constructed in several ways. Perhaps the bones could be glued together or fixed together using pins also made from bone.

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Rebar

If stone mages can really only move stone or concrete and not cause it to flow like a liquid then sticking enough metal tensioning in it could make it impractical to disassemble before archers or whatever ruin your day.

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Build a moat, or rather more of a aqueduct, on top of the wall.

This ring of water will act as an elemental resonance dampener.

This is similar to how, if you build a stone bridge over a stream. Then have a stone mage strengthen it by fusing the bridge into a single stone (top and bottom), it will block the flow of aqua mana down stream. Causing all sorts of ecological problems! Don't do that.

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