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The setting features biomancers who, to summarize, are magic users who can create living things as minions or servants through various ways(seeds, modified eggs, whatever), have the option to directly control their creations like a puppeteer if they have line of sight, and can manipulate life in general, including but not limited to altering their own bodies/cells and the bodies/cells of other living things(if they can come into contact with them).

The effects of their magic can happen either slowly or quickly, the time to completion being chosen by the biomancer and a typical effect being the aquiring of claws, but the faster it happens the more pain is involved in the process. There is a high amount of biological knowledge required to do a great degree of anything in biomancy so it's not like a novice or unlearned will be able to do everything they want, but even the greenest of novice tends to know a reasonable amount of human anatomy and knows you can't live if your heart stops beating so try and stay away from their touch if you fight one.

Biomancers, though they are magic users, are a little more limited to the laws of physics than other magic users, being that their creatures and any biological modifications are subject to the square cube law and nutritional needs and all that, and unless they want to dedicate the time and effort to experimentation they tend to be limited to similar designs to what is already out there in the biosphere somewhere(after having discovered and studied/took them apart to see what makes them tick). The biosphere they have available to them is similar to ours, with the addition of magical creatures whose atypical workings are magical in nature and so won't be able to be able replicated by biomancers as whatever creature a biomancer makes is non-magical in nature.

Magic users in many settings have the ability to significantly and effectively contribute to a war effort or the siege of a fortified location, and so I want this to apply to biomancers as well. Problem is I don't know if they have the ability to do so as they are now, seeing as living things tend to be rather fragile and weak compared to mechanical war machines of basically any era or the good ol' large and heavy ram.

The 'ram' part is what will be the focus of this particular question or, at least, the ability to break down or through the gates of a gatehouse during a siege. Ideally the creature the biomancer designs in an attempt to have it be able to do this should still be alive afterwards in order to deal with other gates beyond the one it first deals with, but a one-shot suicidal creature will do as well, if a living thing is even capable of breaking down those gates, portcullis included.

With the ability to freely design creatures in mind, seeing as that might increase the odds of them being capable, my question basically boils down to... Are living things capable of breaking down a medieval gatehouse's gates?

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    $\begingroup$ How much time have you got to get in? Termites... $\endgroup$
    – Zeiss Ikon
    Commented Dec 9, 2021 at 19:46
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    $\begingroup$ A siege lasts months to years. $\endgroup$
    – Jan Hudec
    Commented Dec 10, 2021 at 7:34
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    $\begingroup$ Not exactly answering the question, but... why break the gate? Controlled organisms can produce much more devastating effects. Stinging or even poisonous wasps; fish or reptile that can poison city's inner water sources/storages; crows that spy enemy headquarters; less intimidating, but historically accurate mice or bugs that devastate their granary; and the weapon of the last chance – plague! (spread by fleas, so the biomancers should be perfectly able to contain it). $\endgroup$ Commented Dec 10, 2021 at 7:43
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    $\begingroup$ some (obscure but on topic) reference: en.wikipedia.org/wiki/Geneforge $\endgroup$
    – Vincent
    Commented Dec 10, 2021 at 10:37
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    $\begingroup$ How heavy would a human need to be to walk through a brick wall? - about the size of an elephant. Put seven tons of angry on the other side of anything and I'm out. $\endgroup$
    – Mazura
    Commented Dec 10, 2021 at 10:43

15 Answers 15

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Right Tool for the Job

The answer to this kind of broad question is usually some flavor of "it depends." In this case what it depends on is how strong the defenses of the fort you are sieging are, and how long you have to prepare. Luckily the first bit of info should be relatively easy to discover, so as long as you know you are going to be attacking a fort you should be able to give your biomancers enough time to grow you a proper siegebeast.

In general, your biomancers are going to require more time and planning to provide a tactical advantage than just having a war wizard slinging fireballs on the battlefield. But the trade off is that with enough time and resources they should be able to solve any problem in their way. What that solution looks like is going to depend on the fortifications you need to break through, although you can imagine some common themes.

  • Gatecrashers - Cross between a horse and a rhino, their most defining feature is a flat "face" which lacks any sensory organs. They are designed to be pointed at lightly fortified gates/walls and then act as living battering rams.
  • Climbers - Modified monkeys which can rapidly scale most fort walls and are commonly used for nighttime raids. Long tails are designed to be used as ropes for raiding parties and specially designed muscles lock the climber in place as a sort of living grappling hook.
  • Breachers - Used against heavy fortifications or when waiting out the defenders is not an option, these creatures are little more than living bombs. Their stomachs have been redesigned to produce and store volatile chemicals which cause a strong reaction when mixed. A few well-placed breachers can break open even the most stubborn of castle gates.
  • Plaguebringers - One of the simplest but most heinous uses of biomancy, plaguebringers are little more than common rats infected with magical diseases which are designed to spread disease throughout the defenders. Due to the inherently chaotic nature of biowarfare, use of these creatures is considered a war crime by most civilized countries and violators risk diplomatic censure if not outright agression.

War Stuff, but Magical

Looking over the list above, you can see how each of those magical creatures can be used to replicate something that was already done during a medieval siege. Battering rams, sapping, scaling walls, spreading disease. How you siege a castle or fort does not really change when you introduce biomancy, just the tools that you use to do it. So start by figuring out what it would take to win the siege in general and then you can work out how to grow an animal to do that.

Do you need to starve out the defenders? Make super rats that grow and breed extremely quickly and let them loose inside to consumer as much food as they can. Enemy moat causing you problems? Time to grow some large fish that can be hooked together to form floating bridges or platforms. Specially designed moles can be used to dig tunnels or collapse walls, birds can be made to attack defenders on the wall or air drop other creatures inside the fortifications. Even without breaking the square-cube law to make giant living siege towers, magic and some ingenuity can turn biomancers into absolute nightmares to defend against.

Recommended Reading

In general, I would suggest you look up "biopunk" as a genre for more ideas on what your biomancers can do and how they can do it. For a pretty decent look at what mass biomancy might lead to I would recommend the web serial "Twig" by Wildbow. It is fairly long and while their are not many scenes of large scale battles, there are at least a few. Plus the whole story focuses heavily on what the world would look like if something like your biomancers existed. It is one of the better pieces of work I can think of that really explores the potential of biomancy, both the positive and negative aspects.

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  • $\begingroup$ I guess this answers most of my future questions regarding biological sieging as well, even if I'm slightly saddened to already know siege towers won't work due to the SCL, and even if I may have to resort to ever so slightly equipping what might amount to a rhino/elephant hybrid with something that'd aid them in focusing their crashing through a gatehouse if chemically reactive 'breachers' aren't used, as was pointed out in William Walker's answer. $\endgroup$
    – Rubrikon
    Commented Dec 10, 2021 at 20:06
  • $\begingroup$ @Rubrikon For larger creatures I would suggest looking up the man-o-war jellyfish and seeing if something similar could fit in with your view of biomancy. Basically instead of 1 large creature you make a collective that all work towards a common goal. Doesn't remove the pure physics of weight-to-volume issues, but might help with things like breathing or sensory processing. It is your magic system, abuse it how you see fit. $\endgroup$
    – D.Spetz
    Commented Dec 10, 2021 at 20:37
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    $\begingroup$ @Rubrikon Siege towers are made of wood. Wood is biological. Therefore, a biomancer can make a living siege tower. In fact, even if you stay inside the animal kingdom, many sauropods were several times taller than your average castle wall; so, an armored sauropod with stair-like scales on its back could certainly be used as a siege tower. $\endgroup$
    – Nosajimiki
    Commented Dec 10, 2021 at 23:01
  • $\begingroup$ @D.Spetz Many good points, but "Breachers" do not have any good reference points in actual biology. Closest to exploding biology you will get will either be honey ants or maybe sandbox tree fruits, neither of which explode with nearly the force you would need to take down a major fortification. $\endgroup$
    – Nosajimiki
    Commented Dec 10, 2021 at 23:05
  • $\begingroup$ @Nosajimiki What about a creature that produces large amounts of the chemicals that the bombardier beetle uses and ruptures the membrane between the chemicals in death, would that cause a sufficiently volatile chemical reaction? $\endgroup$
    – Lemming
    Commented Dec 11, 2021 at 10:23
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Depends on the gatehouse design, but the answer ranges from "absolutely, like butter" to "It might, if you're able to push the animal beyond it's normal limits."

Another answer mentions termites to go through wooden drawbridges/doors.

Of course, an iron portcullis is a whole other story. So let's look at nature's battering ram: The rhinocerous

Stats we care about: top speed (maximum kinetic energy) and mass. That's ~30mph, and 2.25 tons

There's maths in the comments, but the tl;dr is that you can make a rhino-stopping portcullis out of wrought iron. So to get the job done the first time, you'll need to apply some primitive siege engineering.

If you can fully puppet-control the rhino, get two of them and yoke them up to a proper battering ram that focuses the full power of their charge into a piercing point. This scheme, using human-supplied power, can break such gates, all you're doing is changing where the power is coming from.

For a more subtle approach, bypass the gatehouse and have a team of angry badgers undermine (literally where the term comes from) the wall itself.

Brute animal strength is probably enough: but if you combine it with some clever siege engineering, you've got some truly horrifying options.

Hell, if I had elephants to crank a catapult's winch, or set a trebuchet.... etc.

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  • $\begingroup$ You've applied these formulas incorrectly. 1/2mv^2 is a quantity of energy (measured in Joules), not force (measured in Newtons), and you need to use SI units (kg instead of g). The rhino has 179.56 kJ of kinetic energy. The amount of force required to stop the rhino depends on the duration/distance of impact, and can be calculated with W=Fd. If the door flexes by 0.1m, the impact force is 1.8 MN, which is an order of magnitude too low. $\endgroup$ Commented Dec 9, 2021 at 20:40
  • $\begingroup$ Wrought Iron will tend to break, rather than bend, especially if cold and hit hard enough, but you're right I mixed up power/energy. When I re-run the math, 1.8MN over that point of impact is roughly 80MPa, or half what you'd need to break through it - rather than an order of magnitude low, but I'll adjust to focus on the need for equipment to finish the job. $\endgroup$ Commented Dec 9, 2021 at 21:04
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    $\begingroup$ @WilliamWalkerIII, don't forget the rhino will flex quite a lot itself, limiting the peak force it can deliver even to a very hard material. A battering ram with iron point is much better at this. $\endgroup$
    – Jan Hudec
    Commented Dec 10, 2021 at 6:20
  • $\begingroup$ @JanHudec That's why I recommend precisely that in my answer? $\endgroup$ Commented Dec 10, 2021 at 14:12
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    $\begingroup$ @WilliamWalkerIII You are mistaking wrought iron for caste iron. Wrought iron is a low carbon iron alloy which is softer than caste iron, but can bend quite a bit without breaking. And, depending on what century we are looking at, if your gate smiths know that biomancer summoned siege animals are a risk, they would have likely opted for the more expensive tempered medium carbon steel which could be depressed by several feet and then just spring back to its original shape. $\endgroup$
    – Nosajimiki
    Commented Dec 10, 2021 at 14:44
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Use a Tree

Tree roots are designed to push thier way through solid Earth as they grow. This includes breaking apart and displacing solid concrete and brick work as many property owners have had to learn the hard way. Rather than trying to make an animal that can survive crashing into the gate quickly, injuring itself, you should instead focus on the slow and powerful forces a tree can bring to bear.

The Portcullis is really easy. Most portcullises were not locked down, but instead relied on thier weight to stay closed. Depending on its exact size and construction method, most portcullises would have likely weighted anywhere between 1-10 tons. Likewise, most mature trees weigh between 1-10 tons and needs to be able to support several times its weight to be able to survive wind; so, it is perfectly feasible that making 1 or more trees grow up under the portcullis would be enough to lift it open.

As for swinging gates, I have a gate in my back yard that literally ripped open by vines that managed to grow between fence and gate and force everything apart. Normally it would take decades and really bad grounds keeping for plant growth to break through a castle's defenses, but since your biomancers can accelerate growth, They can simply raise what they need from the ground.

enter image description here

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    $\begingroup$ Totally agree. I just had to fork 10k€ to patch a stone wall in my new house, which was completely ripped apart by... ivy! Yeah, I'm not kidding $\endgroup$ Commented Dec 11, 2021 at 15:01
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Pneumatics

I'm surprised no one else mentioned this, given your stated abilities include:

altering their own bodies/cells and the bodies/cells of other living things

Pneumatic/hydraulic force is frequently underestimated. Plants already sort-of do this and can absolutely destroy a castle. All you need is a good pump and a "skin" that can handle some pressure. This video ought to give you some ideas. The trick, in any case, is to find the weak points and force them apart.

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Ground sloth.

sloth dig

https://www.sciencealert.com/this-massive-tunnel-in-south-america-was-dug-by-ancient-mega-sloths

Megasloths dug this tunnel. That is stone. The ancient sloths were big and had big claws: diggers extraordinaire. They could dig through the door.

If only extant animals count, a motivated bear could probably also dig through a door in a day. A badger could probably also dig through a door but they are small and so would make only a small hole.

I insist that when the biomancers summon their door digger, they pose menacingly and bellow 'MEGASLAAAAAAAAAAAWTH!" Then there is lightning and stuff. Then the sloth shows up.

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The gate isn't the problem, the guars are!

If the gatehouse is not guarded, breaking in is not hard. A portcullis can be simply jacked up with help of levers, a drawbridge can probably be unhooked from the chains if you can bring a ladder and climb up, a door can be chopped up by axes… The reason the attackers can't do that is that the defenders will be shooting at them, pouring boiling oil on them and doing any other nasty thing they can to keep them off the gate. So what you really need is to disable the guards.

  1. If you can control pathogens, that's probably the most efficient option. Pathogens is the one thing medieval people have very little defence against.

    Note that shooting dung and rotting carcasses into a castle with a trebuchet in the hope of starting a disease was a thing in sieges.

  2. Another option is insects. A huge swarm of angry hornets is another thing the guards have very little defence against. They can hide inside, but they need some opening to shoot or pour hot oil on you and the hornets can get at them through that opening.

    So if you have control over them to prevent attacking your own, they are better than covering fire. They can allow your engineers to come to the gate and disassemble it in any suitable way.

  3. Massive woodworm infestation can also do some good in a long lasting siege.

    A siege means the attacking force blocks all access to the castle and waits until the defenders eat all the supplies and surrender because they are no longer able to sustain themselves. Which takes month to years. There is basically no fighting, everybody is just sitting around and waiting who runs out of supplies first (the attackers can bring supplies easily, but it's still expensive).

    Infesting the wooden parts of the castle—the roof frames, furniture, wooden lining of rooms, barrels holding the supplies and such can make the castle uninhabitable, persuading the defenders to surrender sooner.

  4. All large animal options are basically just getting cheaper labour.

    Humans have dominated the Earth because humans with appropriate tool are better than any animal in almost anything they want. But soldiers have the disadvantage of demanding pay in addition to food, so if you can control some animals that only need feeding, you can make the operation cheaper, which means you can maintain the siege longer.

    You can use some burrowing animals for undermining the walls (conclude with good old gunpowder explosions one the burrows are dug), and you can use whatever dangerous animals as guards on the paths to the castle for maintaining the siege.

    What is most dangerous depends on the precision of control you've got. A chimpanzee with a spear would be significantly more efficient than a tiger. A tiger has short reach so a human soldier with a spear can pierce it before the tiger can touch the soldier, but if a chimpanzee can wield the spear with some efficiency, it has the same reach and more strength in a smaller package.

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Creatures capable of eating wood already exist, be it termites or furniture beetle

The female lays her eggs in cracks in wood or inside old exit holes, if available. The eggs hatch after some three weeks, each producing a 1 millimetre (0.039 in) long, creamy white, C-shaped larva. For three to four years the larvae bore semi-randomly through timber, following and eating the starchy part of the wood grain, and grow up to 7 millimetres (0.28 in).

enter image description here

If you boost them with same magic, there is no medieval door which can resist you.

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  • $\begingroup$ Carpenter bees bore holes (but to live in, not to eat the wood). $\endgroup$
    – Zeiss Ikon
    Commented Dec 9, 2021 at 20:02
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    $\begingroup$ @LDutch that gate has to fall apart in 2 days.. I propose to upscale the operation, deploy a magic woordpecker, or a magic beaver family. $\endgroup$
    – Goodies
    Commented Dec 9, 2021 at 20:46
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    $\begingroup$ "there is no medieval door which can resist you" <- unless the door is made of iron While most larger portcullises were iron only iron-reinforced, many smaller ones were pure iron because they needed the extra weight to make sure they could not be lifted by hand. $\endgroup$
    – Nosajimiki
    Commented Dec 9, 2021 at 23:00
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Perhaps your biomages can find a way to make teredoes (aka shipworms) live on land and work faster than they naturally do.

If you go to a rocky part of a seashore, you'll likely find rocks with holes in them -- holes that start small and get larger as they go deeper. Sometimes, you'll see what looks like a clamshell in the hole. Those holes (in rock) are made by shipworm. Throughout the age of sail, they were the major limiter on the lifetime of a ship hull; too many of them bored all the way through, and the ship would leak faster than you could pump the water out. And as noted, they don't just bore in wood -- their natural home is bored into rocks like the ones you find on beaches with holes right through. They can also bore bronze and wrought iron (though I gather they have trouble with actual steel).

The limitation, as found in nature, is that their boring rate is very slow. They can bore in rock because they bore with the edge of their (clam-like) shell, and replace the shell material (nacre or similar calcium compound) as it wears away. The holes get bigger as they go because the animal grows.

If your biomage can get them to live at a hundred times their normal rate, and supply a source of nutrition that will let them replace the shell fast enough, you wouldn't need to break down a gate or portcullis; they could tunnel right through a stone wall!

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Termites are living things, and so is wood rot - it's a kind of fungus. Magic termites or magic fungus could presumably eat wood at a much faster rate.

Harder materials like iron and stone are harder to get through. In this case I think the better option is not to eat through the materials, but to apply enough force to bend the iron or crack the stone. The roots of large trees are able to do this, by moving the soil underneath the building. So I'd go with magic trees that grow very quickly.

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    $\begingroup$ Fungi can move pretty fast given a good store of energy. After a year of growth inside old branches, they can produce fruiting bodies - mushrooms, or toadstools ("that which was shat by a toad" i.e. not recommended eating) overnight. Perhaps your wizards can breed ones capable of growing mycelia and colonizing new wood equally fast. $\endgroup$ Commented Dec 10, 2021 at 16:03
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Many non-magicked animals have evolved with ramming in mind. Rhinoceroses, Triceratops, goats (the latter you might want a herd of). Hippos do not ram ordinarily, but if convinced by a biomancer could create an incredible amount of force by running full speed (30 mph) and then performing a side slam with their full weight (3000-4000lbs).

Another form of life which can exert a tremendous amount of force, albiet over a longer time period, are the growth of trees/roots. I could imagine a biomancer sculpting a root structure of some kind to bend an iron portculis completely out of shape over the course of a day or two, being powered by some nearby biomanced trees.

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One option not mentioned yet is biological bombs. It'd take some preparation to come up with the right creature design, but I think it should be possible to make a metabolism that produces highly energetic chemicals which are then stored in some sac somewhere, to be ignited upon need. Maybe you'll need some bone structures to create a pressurized vessel, I don't know much about explosives. 😅 But, say, methane is a highly flammable gas that is already being produced in vast quantities by life as we know it.

Alternatively, if making it go boom is too hard - biological blowtorches/flamethrowers? Can you say "dragon"?

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    $\begingroup$ Metabolism can also produce a highly caustic acids. $\endgroup$ Commented Dec 10, 2021 at 7:39
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    $\begingroup$ Man... I wonder what large amounts of bombardier beetle chemicals will accomplish 8| $\endgroup$
    – Lemming
    Commented Dec 10, 2021 at 7:56
  • $\begingroup$ @Lemming - Probably nothing good. 😆 $\endgroup$
    – Vilx-
    Commented Dec 10, 2021 at 8:09
  • $\begingroup$ @VictorSergienko - Acids are good against metals, but I wonder if they aren't too slow-acting for a battle? If you have plenty of time though - sure, no metal will bar your way. $\endgroup$
    – Vilx-
    Commented Dec 10, 2021 at 8:12
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There are several real-world creatures (alive and extinct) to draw inspiration from:

  • Elephant - elephants are one of the most obvious, they're large and have been seen break down and uprooting trees in the wild - depending on how the gates are constructed - a reinforced wooden door would probably break much like a living tree. Some gates in the Indian subcontinent still exist that were designed specifically to defend against attack by war elephants, studded with long, sturdy spikes.

  • Ankylosaurus - the ankylosaurus had a thick skull, bony plates along its back and, most notable for breaking down gates, a nearly two foot thick bone club at the end of its tail, that massive tail is believed to be able to break other dinosaur's bones in a single hit, and could do damage to a doors and gates in the same way.

  • Water Buffalo - these are a little more questionable, but they have one of the most strong and sturdy sets of horns in the animal kingdom, with many having a relatively solid 'forehead' of horn before they sweep back.

It's certainly possible for animals to break down doors and gates, and some kinds of walls - and like what happened with war elephants, specific defenses may be designed to counter that attack.

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Weaken the iron bolts that hold the wood in place with tons of acid. Possible designs include red ants that spit formic acid, leading to the formation of iron formate.

While at it, anything that can cause the iron to rust fast is handy to attack the hinges where they are embedded into the wall and then make the joint crumble under the pressure of expanding rust. The application of a weak solution containing chlorine iron (table salt or digestive liquid - you need them in catalyst quantities) then plenty of oxygen and moisture will do wonders (aka oxide jacking) (call your weather mages to provide a thick fog for the moisture).
This will work for the bolts that hold together the wood in the gates, though it won't be the jacking action but just the simple mechanical weakening of the fasteners.
For portcullis, just rust (at accelerated pace) the horizontal beams in the middle of them.

With the weaknesses created, plug in whatever mechanical force you need to bring the gate or portcullis down - depending on how good the "rust warriors" job is, you may not need too much.

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Use a tree (less elegant version):

Plant a tree "in range"[1] from your target. Grow the tree, possibly with a slight inclination towards the target. When its height is sufficient, topple it on your target, smashing it to pieces.

[1] A quick search suggested about 130 meters as maximum stable height for trees.

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Ivy

Ivy and other similar plants grow their roots wherever they can find a crack. Those roots then expand, widening the crack.

If you drastically speed up the growth and magically replace the water and nutrients that the plant needs to grow, you can take the time needed from decades to as short as you want depending on how much energy you want to throw at it.

Also, by cutting off the nutrients and water before you cut off the growth, you can have the plant die and desiccate itself. Otherwise, you've replaced an impenetrable gate/wall with an almost as impenetrable plant.

Note that it also dissolves the rock that the roots are anchored on. So that will help take down the walls if that process is sped up or inhanced.

Fire might be an issue. Maybe use a cross with an ice plant. Iceplant is a succulent. So, it holds enough water that it doesn't really burn unless you apply enough heat to first boil off that water.

If you just want to tumble the wall, then anything in the ficus family will do it. The strangler fig is only the most notorious example. Any ficus built next to anything is a danger over time.

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