Useful (late-)medieval Weapon against Iron Buildings?

Question

I'm going to jump straight into a very small description of the world my first novel-thing is build around.

It's an Earth-like planet orbiting a binary star. Climate is slightly more extreme than on Earth (think colder winters, hotter summers) and the ~800 million inhabitants adapted quite well to it. Technology is on a late-medieval level, compare it to ~1400-1425. Magic IS a thing on this planet. Everybody can learn to use it to the same extent, though it requires a lot of studying and memorizing runes. The strongest incantation documented was cast by a famous scholar almost a thousand years ago and since then, people came close, but could never reach that level. What was the incantation? Well.. he summoned an orb of fire the size of what we on Earth know as a Hippopotamus. That's it. Magic can levitate small objects, too, but its nothing too frightening or too dangerous at any time.

My "Villain", the Protagonist, manufactured with some aspects of Peter's Evil Overlord List in mind, can summon a fireball the size of a Chihuaha. It takes about 8 hours to prepare and exhausts him. So he does not use it. At all.

He'd prefer something that works efficently with the technology that's available to him. Something that is relatively cheap to produce and doesn't require any hand-wavium to construct.

What for? To destroy the main fort of the opposing forces. Or at least damage it beyond repair. There's just one tiny problem: It's mostly made of Iron and Stone.

• The fort stretches over an area of about 8km² and is surrounded entirely by walls that reach heights of 12m.
• The walls are made of stone and have a small core structure made of iron inside of them (so: stone-iron-stone, 6m-2m-6m).
• The buildings are mostly build with the same principle in mind. Build a small relatively small core of iron and "coat" it with stone.
• There are roughly 4.000 men living in this fort, all armed with melee- and ranged weaponry.
• The fort NEEDS to be destroyed, as it blocks a path to the Evil Overlords destination with no other way around it. Preferably in a matter of a couple years or less.

That should be enough (hopefully non-confusing) information. So the core question:

What weapon could my Evil Overlord build and use against this fort with the technology he has available, without using any hand-wavium and without magic?

Answer 1

You could use immense, intense heat as was used against the vitrified forts

This image shows stones from one such fort. It looks like an igneous rock because of the bubbles, but this is in the UK. That rock partially melted.

from archhighland.org.uk

A fort with an iron core in the wall would be extra susceptible to an intense heat attack. Once it gets hot enough the iron itself will burn. When that starts there is no stopping it. This was demonstrated most spectacularly in the 9/11 attacks. It is the reason insulation is sprayed on steel beams. from http://www.sciremediation.com/Web%20Pages/Fireproofing.htm

How did the ancients vitrify the forts? It is an open question and a very interesting one. The vitrified forts are real and no-one knows how (or why!) they did it. In the book Merlin's Ring it was an attack by Atlantean swanships on their rebellious colonies.

On reading up I found this metalsmith who was mad that 911 conspiracy theorist were asserting that steel doesn't burn and so made a video. https://www.youtube.com/watch?v=FzF1KySHmUA

One would not think you could burn down an iron and steel wall but a smith would know. You can have the idea come from your blacksmith. You could do it with a big pile of coal or charcoal against the wall. You could have a tunnel underneath the pile leading a short distance away - the heat of the fire would pull air up thru the tunnel (strongly!) and that fire would burn really hot. The defenders would not be able to get near it. Once it was hot enough to ignite the iron inside the wall that would propagate and the whole wall would go from the inside out. It would glow.

Answer 2

Sappers and... pigs

Just (well, it's not that easy, but still the easiest way IMO) dig tunnels under your walls, put a few dozen oiled fat pigs, set them on fire and watch the wall crumble under its own weight after the prolonged intense heat weakened the structure !

https://en.wikipedia.org/wiki/Early_thermal_weapons#Animal_renderings_and_parts

At the 1215 siege of Rochester Castle, King John ordered that fat from 40 pigs be used to set fire to the mines beneath the keep, which caused it to collapse, a cheap and effective technique in place of the more complicated mixture of sulfur, tallow, gum, pitch and quicksilver he had used in France the previous year.[81] Animal fat was not uncommon as an accelerant; in the 13th century French sortie-parties would often be equipped with animal fat, straw and flax to use as fuel when setting fires amongst enemy siege engines.

Answer 3

Sorry, but this is not a medieval fortress. It's too damned big and it takes too much iron.

Assume for the moment that it is circular. Then it has a diameter of about 3 km. Total wall length is 10 km. With an iron core 2 meters thick and 10 meters high, it uses 200,000 cubic meters of iron. or about 1.6 million tons. Given early iron production techniques, this is simply not feasible. It's hard to be precise, but by 1500 (end of the medieval period), total iron production for all of Europe was on the order of 60,000 tons, so the fortress represents total European production for 27 years - all of cased in stone and unusable for tools or weapons.

Furthermore, if the core is to be any good, it must be formed into a solid structure. Just piling a bunch of iron plates or ingots between 2 stone walls will provide no more protection than a bunch of rocks. This, in turn, is beyond the capabilities of any medieval technology.

And speaking of weapons, the existence this much iron (and it must be cheap to use so much) suggests that iron (and steel) must be widespread, and there's no way steel cannon are not in common use. Once you've got these bad boys running around, high walls are simply irrelevant - unless you can make your impossible iron structure. Even so, the stone facing is a waste of time - it will fall off in sheets as soon as it comes under concentrated fire, exposing the iron core.

So, sorry to be a spoilsport, but your fortress is itself inconsistent with medieval technology, so limiting the attackers to medieval technology makes no sense.

Answer 4

Acid

Midieval chemistry; though most often referred to as alchemy, was already capable of producing several strong acids.

Put this in a container that breaks upon impact but doesn't react with the acid (glass containers) and watch how these acid filled container not just destroy, but dissolve the iron fortifications.

Raw materialistic damages aside, this weapon would be feared as the damage to soldiers and their armor would be substantial as well if they get doused in acid. So you're not only destroying the iron buildings, but it would break the morale of the soldiers as well when they see the buildings dissolve and their comrades get burned straight through their armor.

Answer 5

Fire and smoke...and animal carcasses

But you are not going to target the walls.

In warfare it is generally best to strike the softest target. It does the most damage to the enemy and causes the smallest loss in your own troops.

According to your question you are willing to wait years for the siege to be completed.

So what to do with the items I listed.

Fire and smoke: These two obviously go together. Lob oil canisters over the walls with catapults or trebuchets, target everything, the goal is lots of fire and smoke, not necessarily major damage to the fortress. After all, its a nice fortress a place an evil overlord may want to set up camp later.

With the fire and smoke you could potentially create the fortress version of a beach head on the walls by making the defenders scurry away from the fire and smoke.

Carcasses: If you are the patient type of evil overlord you can simply kill the defenders the slow and painful way (as opposed to the fast and painful way) by lobbing dead animal carcasses over the walls until they all, or most at least end up succumbing to disease...

If you go this route it probably makes sense to keep the fire around for cleanup...

Answer 6

Thermal shock can be your friend! Okay, not totally sure how to make it work on a very large scale, but here is how you might generate a breech. This should be done in the dead of winter.

Start with general bombardment, you want to slough off as much of the stone facade as possible, exposing the iron underneath. You want the iron exposed down low, near the base of the wall.

Protect your minions however you deem practical. Have them approach the base of the wall and build as hot a fire as possible right at the joint of stone and iron. Keep the fire going as long as you can and if possible, get the iron as close to red hot as possible. In the mean time have your forces build a trebuchet and a ballista. Also bring in some method to pump lots and lots of cold water. Get heavy iron arrows for the ballistae and load the trebuchet with blocks of ice.

Fuel the fire first with wood, then with coal, if possible. Your goal is to get the joint between iron and stone glowing red. If you can get the iron that hot, it will begin interfering with the defenders efforts to keep you away from the wall. Keep this going for weeks if you can. Don't actually hit the wall with anything yet.

At the same time, send minions in small parties around to other points in the wall. Near Every joint, have them drive spikes into the stone and pull them out, then fill the resulting hole with water, and retreat. Then, under cover of darkness, get as much water flowing underneath the walls as possible, using sappers and the like. As long as this place isn't firmly on bedrock, you should have some springs nearby that can be diverted to weaken the walls at the foundation.

The defenders will be trying to figure out what the fire is for and will likely ignore a lot of the other activity, since it will be small parties who look like they are running up, banging on the wall and running away.

Since this is winter, the water in all of those little holes is going to freeze and start cracking the stone. If your minions do it right, the aggregate damage is going to severely destabilize the walls. A few shots with a trebuchet will shatter the remaining stone facade. Then when you hit the iron core of the wall, you might be able to create a spalling effect inside, killing the defenders right up against the wall with stone shards.

Now it's showtime for the main assault. With the iron glowing nicely, begin bombardment of the hot spot with cold water, 500 lb iron arrows from the ballistae, and huge chunks of ice. Make a big show of it. If you are lucky, the resulting thermal shock and repeated hammering of heavy objects will tear the iron away from the stone all along the joint.

Meanwhile, after the bombardment of the hot iron wall begins, roll up trebuchet to start hitting areas that you have been driving holes in the stone to let the ice weaken it. If your sappers did their job properly I would predict that you will get a breech in one of those spots faster than your big show. This will have been the aim all along. If you can figure out a way to get a fairly small hole all the way through the iron wall and get a lot of water flowing through it to the gap between iron and inner stone walls You can create a lot of damage that way. it would make your sappers job a lot easier

If the thermal shock works, then you get a neat story to tell and your legend becomes that much greater. If not, you still knock some walls down and then get where you need to go. This should only really take a few months to do. Also, you have a place to keep warm in the winter (hah)

Answer 7

Your'e not going to put much of a dent in 2 metres of iron encased in rock. So go over it or under it, or normal siege stuff like attack the gate or lob some diseased corpses over it. Or all of the above.

The gate or gates is assumedly a weak point. And that's why it was a focus in many sieges. Ideally you smash through the gate or climb the walls or use spies to open it from within. You don't expend too many resources trying to bash a hole in the wall.

Answer 8

Infiltration

An unconquerable iron fort right on the path to your headquarters is bothersome when the enemy has it, but could be really useful if you had it. The thick iron walls of that fort would be incredibly hard to replicate. In fact, I'm pretty sure solid iron or steel structures of that size do not exist in our world now. So it would be a shame to destroy it.

You have some options:

Traitors - you could make people who inhabit it now come over to your cause. How you do it depends on what your cause is an who the inhabitants are. Once you get some of the high command to secretly support you, they can restructure the rest of the fort and come over to your cause in a relatively short time. If this is impossible, it still only takes a few people on the inside to open the gates and let you march right in.

Failing that you can still starve them out, then move in. Block access to their fort, and hit them with biological warfare. Block their food supply, poison their drinking water and try to infect them with diseases. Once there is nobody left in there who can resist your attack, get some ladders and go in.

Or you could conquer the fort through some conventional battle tactic.

If you absolutely have to destroy it, fire and/or undermining are your best bets.
But it would be a waste.

Answer 9

Powdered Combustibles

Fuel-air explosives are relatively cheap and easy to make, safe to transport, and have been known about for a very long time. (Look up grain silo explosions on youtube.)

A cup of finely powdered flour dispersed in the air is roughly as potent as half a stick of dynamite.

Flour's not the only option. In a medieval world powdered coal or charcoal would be a much better choice since it would be a lot cheaper than burning up everyone's winter food supply. Of course, if you've broken into the fort and are using their food supply, the concern goes away.

Delivering the explosive is a bit tricky in combat conditions. You have to get your finely-powdered combustible mixed with the proper amount of air. During Sherman's battle for Atlanta, flour from the local mill was used to blow up buildings, but those were strategic objectives well away from the active fighting. Modern fuel-air devices generally use liquid or gaseous combustibles sprayed from nozzles for reliability and ease of ignition, but building such a device at a medieval tech level would be a challenge.

Nevertheless, an enterprising artificer could probably come up with a way to throw a barrel of powdered charcoal from a catapult and have it burst open over the fort and disperse into a cloud. After that, you just need a way to ignite it before it dilutes enough to stop being explosive.

If you're planning to take the fort and then destroy it so it can't be re-occupied, then just transport the stuff in sacks. A bit of experimentation will let your men know how many sacks of charcoal to how many square feet of building are necessary to blow it off its foundations. Just powder it finely enough that you can shake it out of the sack and have it float in the air.