# Using our current scientific theories, is it possible to develop better siege equipment than medieval ones? (Updated)

Sieges are not a mystery. Bribe someone to open the gate or starve them out are the popular options. And actual assaults are not very common. Sun Tzu would even advice to

And the worst policy of all is to besiege walled cities. The rule is, not to besiege walled cities if it can possibly be avoided.

The armies of the past used clever and scientific methods to assault fortifications. Rams, towers, ladders tied to the ground, raising a mound, sapping the walls, using stone throwing engines, using protected stuff to get under the walls...etc.

Yet one wonders if our current scientific theories in say mathematics or engineering or physicists can actually influence older designs of any older equipment?

And that is the heart of the question. Imagine a scenario in which we have a science loving king in a prosperous kingdom in 14 century Europe world. The king hears of this bright scientist. The king then sends for that person and then as he is the king he commissions him to make him siege engines.

The purpose is that the king is gearing up for a war in the future 2-4 years. The opposing kingdom uses the latest and best fortified cities/castle that Europe had in the 14th century, so the king wants to have the edge against them.

The dreaded budget is basically the king telling him that his 14th century Europe kingdom will offer him vast resources to help him (Much like Urban and Mehmed II) But as they are in 14th century Europe; the genius of that person is limited by materials and people to help him as well as the purpose of siege engines and time and lastly the fact that the king expect him to do something somewhat better.

What is a siege engine?

A siege engine is a device that is designed to break or circumvent heavy castle doors, thick city walls and other fortifications in siege warfare

Now history is full of entire nations falling into the whims of Royalty so I don't think it's complete insanity to imagine this.

That person is very smart and for the purpose of limiting this question [I don't get why letting people be a little creative is frowned upon] He only knows up to our theories in those fields: mathematics, engineering, and physicists. Those is what I imagine are vital in making siege engines. But I could be wrong.

He is then expected to try his best.

If yes do you have an idea? If no can you explain more?

A list of limitations

• 2-4 years of making his stuff.
• Do better than what already exist in 14 century Europe's siege equipment.
• Actually come up with a siege equipment not tell the king to bribe people.
• 14 century technology.
• Wealthy kingdom with vast resources but limited to the time.
• He is only a genius (Up to what we know now) in those fields.

Thank you for all the suggestions and I'm really sorry for those who answered before editing the question, I appreciate the input nevertheless.

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Nov 19 '20 at 16:44
• So, do you want modern science, or 14th century science. And do you mean 14th century or 14 hundreds? They did have geniuses in the past. Leonardo da Vinci was only two centuries away (16th century). Nov 20 '20 at 3:28
• So yes, in fact, someone from the 1200's, or 13th century, could certainly go forward to the 1300's, or 14th century, and tell them how to build a better 'siege engine' called, well, a cannon. Nov 20 '20 at 6:29
• Do you want the 14th century or the 15th century, you say both. 1400's is the 15th century. Nomad already asked but you have not answered.
– John
Nov 20 '20 at 16:27
• "Using our current scientific theories, is it possible to develop better smthg" - would be great to have tag for this. Nov 24 '20 at 0:40

20th Century knowledge.
One person
14th century tech available. Important note: this is 1300's NOT 1400's

The kingdom is prosperous, thus the Great Famine and the Black Death have not arrived yet. So this places our scenario somewhere between 1300 and 1315.

Gunpowder is known, but almost no clue what to do with it. Fortifications are optimized to defend against bows and infantry, and to serve as cavalry bases for sorties. Iron is common. Wrought iron is a novelty, not really explored yet. Steel is known of but not understood at all.

Here is a quick action plan for your inventor:

• Make nitroglycerine, and promptly safe it with diatomaceous earth, making dynamite. This is easy, as all it requires is nitric acid, glycerin, alcohol, and a disposable number of worker to do the stirring, draining and purifying. In a production facility far, far away from anything valuable. All of the feedstock chemicals are known and available in the era.
• Draw up the specs for a large-bore low-pressure brass cannon. Give this to the royal metalsmiths. With supervision, they should be able to cast a reliable mortar-type cannon within 2 years. Unfortunately we won't be able to load our dynamite in the cannon, it is too shock sensitive. But it is a big, expensive, brass thingy that will impress the King very much.
• Make a basic montgolfier-type hot air balloon, to use for scouting and enemy observations.

For actual sieges:

• Trebuchet-launched Dynamite packages
• Brass cannon firing solid shot
• Tunnels, with explosives once under the walls.
• balloon(s) for observation

# OK, that's the flashy stuff, to satisfy the King.

Here is what your timetraveller genius actually does:

• "invent" wheelbarrows. Yes, they didn't know about them(despite the Chinese having used them for 2000 years by then). This about doubles productivity of miners, and helps in many other sectors. This hugely boosts industry.
• invent a simple cypher, allowing the king's messengers to carry messages around with no chance of the enemy reading or faking them. This hugely boosts military and financial intelligence. Yes cyphers were known before this. But they were not in common usage because they were not at all secure. Modern techniques can make very simple public-key cyphers that are easy enough to work with, but cannot be broken or faked without some serious mathematics. Combine the cypher with a system of Semaphore towers for rapid communication over distances.
• invent basic sanitation and hygiene theories. Teach people that crap in the streets not only stinks, but actually causes illness. That cats are good, because they kill rats, not agents of evil. That a simple wash with water and soap before surgery on your wounded soldiers will better than triple their post-op survival rate. This hugely boosts general health and resistance to diseases.
• Make and bottle penicillin extract. Do NOT tell anyone it is extract of mouldy bread. Say it is a "Blessed Tincture of Rare Oils Blessed by a Touch of a Holy Saint bearing a splinter of the True Cross", or some similar pseudoreligious doubletalk.

With any luck, your intervention will allow your kingdom to sidestep the Great Famine, and the later Black Death Plague, thereby ensuring their supremacy over the entire region.

• Reality Check: The time traveler is either hanged or recruited and locked away by the Church because they threatens their power. Especially if they goes around selling relics. Otherwise, good answer. +1 Nov 20 '20 at 13:05
• @Mindwin Well yeah. That happened to some of our previous agents. Galileo got all scared and retracted his statements. Socrates got poisoned. Clever, they framed it to look like suicide. That jesus guy really got nailed! Only Leonardo managed to pull off the "eccentric genius" persona, and it took centuries for his genius to be fully appreciated.
– user79911
Nov 20 '20 at 13:12
• So you were the guys that created that clusterf--k paradox that stops every one else from traveling back in time-space anywhere near middle-east from 50BC to 150AD. Good to know. Could you tell me the Minkowsky Coordinates of your dimensional apparatus so I can send a strike team back in time to friggin' fix it? Nov 20 '20 at 13:19
• It's already been fixed, next year.
– user79911
Nov 20 '20 at 13:20

Stuff a modern engineer can do which will quickly reduce any late medieval fortress:

• Dynamite and TNT: any self-respecting chemist can make them, and they would be devastating against late medieval walls and gates.

• Shrapnel shells (launched by catapults if you don't like cannon) which will quickly convince the few maimed survivors of the defense force to surrender.

• Steam-powered bulldozers which will be practically immune to any defensive weapons and will quickly undermine the walls.

• Real incendiary projectiles (napalm or phosphorus, your choice). There was lots of wood behind those walls.

In fact, given that the task is to devise siege methods against 15th century fortresses, you don't even need a "modern" engineer. All you need is a Renaissance-era military engineer well versed in the art of siege: by the late 1500s they had developed a comprehensive theory of approaching the walls with a triple line of trenches (called "siege parallel"), with the goal of digging tunnels under them, filling them with gunpowder and making it go boom.

Late medieval style fortresses, with their romantic high walls and lofty towers became utterly obsolete by the late 1500s; there was a very good reason for that. Bringing in technological knowledge from the 21st century is overkill; even technology from the 18th century would be crushing.

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Nov 22 '20 at 4:19

Trebuchets are still used in combat! They were used in the Syrian civil war to throw bombs and in Ukraine during the Hrushevskoho Street riots! They're also widely used to teach physics and engineering and are popular among hobbyists, so many improvements have been made since medieval times!

Floating arm trebuchet: https://youtu.be/UzdSB1znJ2k

This guy has been optimizing everything on a trebuchet. Here he uses a "whip it trebuchet": https://youtu.be/-gn2RGPqe_A

A wiki for such things: http://thehurl.wikidot.com/trebuchet:trebuchets

Also note that DaVinci's catapult was designed after the medieval period.

Kinda tangential, but I would like to see someone use a flywheel to power a stone-slinging pitching machine. That's about the only way to improve on a ballista for anti-air that I can think of (without gunpowder). Such a device would also be faster firing than any extant seige weapon if you used a hopper to funnel the stones between the wheels.

• "if you used a hopper to funnel the stones between the wheels." - wouldn't that jam way too often, possibly with violently disastrous results to the user? Nov 19 '20 at 11:09
• Not if you cut the stones to size like they did with early cannons. Maybe you could use a rock tumbler to get them to the rough size and shape and have your cheap labor smooth it out. If it does cause a few issues it's not the end of the world because life and labor was cheap then. Nov 20 '20 at 6:46
• Why would someone in the 15th century go to such trouble? They already had gunpowder for 200 years. Nov 20 '20 at 14:49
• @JustinThymetheSecond the OP is not clear what time period they want, they give both the 14th century four times and the 15th century twice. Also gunpowder is not the same thing as siege weapon cannons, which did not exist until the start of the 15th century.
– John
Nov 20 '20 at 16:36
• The original version of the question seemed to be asking for solutions that shy away from gunpowder, so I gave the solution I thought would be easiest to enact without having to consider resources or education of the locals. Trebuchets were used in the seige of Burgos (1475) and seige of Rhodes (1480), so our significantly improved trebuchets wouldn't be that out of place, especially since they would cost next to nothing to produce compared to cannons. Nov 20 '20 at 16:45

Will that person, using our current theories, be able to produce better siege engines or improve what already existed?

Aye, and therein is the rub.

"that person'.

Just one person. No army of engineers, technicians, teachers, or tradesmen. No one else has any clue as to manufacturing techniques, craftsman skils, chemical experience, or even knowledge.

"Get me ten kilograms of phosphorous.'

'Yes. m'Lord. But if you please, Ummm, what is thing called "phosphorous"? Where do I find it? How do I recognize it? Oh, and what is a kilogram?'

How do you translate our knowledge into terms they would understand? And what is the life expectancy of 'this person'?

First, this person would have to teach a team of scholars, who would then teach others. Not just knowledge, but skills and methods, terms and definitions, materials identification.

Even something as simple as thermite - extremely useful for creating extremely high temperatures capable of melting and welding steel and iron. Made of aluminum powder and iron oxide. Today, rally basic tuff, easily obtained. But they didn't even know what an oxide was, or even where to look for aluminum, let alone refine and process it. He would have to first teach a small cadre of associates before he could even discuss his ideas, let alone have anything built.

Knowledge alone is not sufficient. You need people with the abilities to use this knowledge in practical ways.

And reacall what happened to Capernicus. Run against the scholars of the time, and all the best knowledge you can muster will not save your hide. You have to convince the ones that will implement your knowledge of its veracity. Without, of course, being called a witch, evil sorcerer, or devil. Can you imagine someone with thermite showing up at the court? They would either worship you as a god, or kill you as the devil.

Methinks diplomacy and politics are a very big part of the equation, and can not be dismissed. This person would need to have a complete understanding of their culture, customs, and political power structure as well.

But if he DID manage to make it, a huge thermite explosion in front of ANY beseiged city would reduce the defenders to whimpering mush. It would be seen as the power of the gods. It scares the shite out of even today's students when the teacher demonstrates even a minute amount of it. No need to break down the walls, and really no super-advanced bewildering technology necessary. Only the ability to identify iron and aluminum, and some basic metalutgical skills to process it. Just the THREAT of being able to harness such power.

Same with amonium nitrate. Easy to make from commonly available household chemicals. Recall that it was nitrate fertilizer that blew up an entire port complex, and left a huge crater. Very big badda boom. Wait - they did not have these 'commonly available household chemicals' back then, so first he would have to teach them how to make these echemicals out of other chemicals. And, of course, teach them how to make THOSE chemicals. See a pattern? What is common to us, is not so common to them. He would for sure be seen as a wizzard, a sorcerer, or whatever they called them where he was. But he would certainly need to be a very good teacher.

Theoretically, he could know how to build a nuclear weapon, and they could build one. But first he would have to teach them metalurgy, engineering, physics, manufacturing, uranium mining and enrichment. After that, they could easily build one. But with adequate knowledge and education, they could build nuclear power plants as well. In a hundred years or so, to build the education system first.

This question is so open-ended in scope, it is difficult to answer, but at the furthest end of the scope it is an unequivocal 'yes, but'. At the other end, the 'but' becomes 'But then again, he could be put to death as soon as he opended his mouth'.

• Comments are not for extended discussion; this conversation has been moved to chat.
– L.Dutch
Nov 21 '20 at 5:29
• @L.Dutch-ReinstateMonica : what about pointing out complete historical falsehoods in the answer, which are tangential to the topic anyway, and would not hinder the main reasoning of the answer at all if removed or corrected?
– vsz
Nov 21 '20 at 15:46
• @vsz "You have to convince the ones that will implement your knowledge of its veracity." And exactly what is the 'historical falsehood' that is tangential to the topic? That even someone as well respected and influential as Copernicus struggled with having his ideas accepted by the vested interests of the Church? 'This person' is not going to waltz onto the scene out of nowhere and 'rock the boat', especially in an area as conservative as warfare. Nov 21 '20 at 16:10
• @JustinThymetheSecond : the historical falsehood is that running against scholars of the time was a mortal danger, the implication of what "happened" to Copernicus (nothing happened), and the allegation of showing some new chemicals will end up in being worshiped as a god or killed as the devil. BTW, during that time period in an "area as conservative as warfare", new and new methods and technologies were introduced quite rapidly. Just look at the development in armor, guns, fortifications. If what you were saying was true, alchemists would have been persecuted instead of sponsored.
– vsz
Nov 21 '20 at 16:57
• @vsz "Alchemies are here prohibited and those who practise them or procure their being done are punished. ..., and they shall be considered criminals. If they are clerics, they shall be deprived of any benefices that they hold and be declared incapable of holding others.' Pope John XXII's decree against alchemy alchemywebsite.com/papaldcr.html Yep. the church just loved alchemists. Nov 21 '20 at 22:22

Modern (i.e. 21st century) theories and engineering are a bit too modern, relying on too advanced resources and machinery. 'That person' better be a military history buff with some hands-on experience (e.g. reenactment). Otherwise, 2 to 4 years is not enough time for the necessary research to bootstrap the technology. You can introduce the scientific methdo itself, but it will pay off in 49 years, not 4. Advanced metallurgy will enable many other advances (better cannon, for staters) but you have to redevelop everything from mining to smelting, and unless you know exactly where the mines are and what to do with the ore, it might take 20 years...

However, there are still some possibilities:

• 14th century is a good era to "invent" the telescope. Not the decisive tool for a siege, but very nice for other types of warfare
• Communication. Introduce Morse code and a heliograph. Again, less useful tactically for a siege than strategically.
• Germ theory. Order the soldiers to dig the latrines at least 100 paces from the camp and "I'll rip your ass if you shit elsewhere! And wash your hands before eating! Yes, I know it is silly, just do it!" and it will be a game changer, compared with a typical 14th century campain.
• I was thinking about Germ Theory (in the context of Time Travel) last night. The conclusion I drew is that without more efficient farming techniques, what "wash your hands and dig the latrines far from the drinking water source" does is just make a lot more starving people. (Remember that the technology of the era was barely above subsistence level.) Nov 19 '20 at 20:04
• @RonJohn At the level of society I agree with you. However, this is about protecting troops in a siege, there it would help. Nov 19 '20 at 21:06
• @LorenPechtel think Unintended Consequences: the soldiers would bring these miracle techniques back with them. Nov 19 '20 at 23:34
• @RonJohn Whatever he does that's going to be a problem. Nov 20 '20 at 4:00
• @LorenPechtel makes me think of Bradbury's "butterfly effect". Nov 20 '20 at 4:33

### You can supercharge a biological siege using modern knowledge.

The purpose of the siege is to capture the city, not destroy it. Starving, poisoning, or infecting were the traditional approaches. We can do those things better with modern knowledge, yes.

Disease often killed more troops than combat: So the first thing I'd use modern knowledge for is infection control - keep my troops away from the city while horrible biological / chemical things are happening. My troops have clean water to drink, and soap for washing their hands regularly.

The second thing I'd use modern knowledge for is to isolate and gather a bacterial infection. I'd suggest Anthrax, as it can be extracted from Cows. Following what's on the wikipedia page one could grow and multiply Anthrax into a large quantity.

Aerosolised anthrax with 20th century or earlier treatment had a fatality rate of 90% - 97% when inhaled. That will wipe out your enemy.

Anthrax can be burnt or killed with formaldehyde, which can be produced in a medieval setting if you've got a good blacksmith working for you making something that can be heated for 650 degrees C (low quality wine -> distillation of methanol in reflux still -> heat methanol vapour in presence of silver).

I'd expect the 3 in 100 survivors to flee and surrender, so you'll have a nice collection of prisoners of war to put in harms way for a speedier decontamination. Give them a bucket of formaldehyde and instruct them to clean the entire city in return for keeping their life.

If they don't surrender, you'll need to fight 3-10% of the enemy army, weak from lung infections. Your side will of practiced fighting in masks, know to wash their hands, etc.

• You can't clean the dirt. Anthrax spores in soil (and most places you might want to capture don't have paved surfaces) will contaminate a place for at least decades, which if you're wanting to move in right away is kind of counterproductive. Nov 19 '20 at 2:52

Production techniques for hydrogen were discovered in 1671. This opens up all sorts of options for flying over the walls.

You could combine with paratroopers and airdrop ideas, so that the siege platforms are never even close to being exposed to arrows and spears.

TNT was discovered in1891, but is readily producible with 14th century elements of glycerine, aqua fortis (nitric acid), and oil of vitriol (sulfuric acid). A little bit of that goes a long way to sapping walls.

• Saponified tallow, aqua fortis, and oil of vitriol gives you nitroglycerine, not trinitrotoluene.
– Mark
Nov 19 '20 at 23:19
• Excuse me. And diatomaceous dirt Nov 19 '20 at 23:25
• That gets you dynamite, not TNT. Unlike nitroglycerine, where you can just mix the ingredients and hope you don't blow yourself up, TNT requires relatively advanced synthesis techniques.
– Mark
Nov 19 '20 at 23:26
• Good to know. I always thought dynamite and TNT were synonymous, and never really thought about it Nov 19 '20 at 23:31
• TNT was a very stable form of exlosive, nitroglycerine was extremely unstable. It takes a LOT of training to handle it, and thus the 'splat' factor - would this person kill himself before he taught anyone else how to make it? Nov 20 '20 at 5:56

hot air balloons

The first ones were made out of paper. Which was already present at medieval times. Sure, your balloon won't use nylon, there will be holes, but it is only about maintaining a temperature difference between inside and outside the balloon.

I found a nice article with simple physics on the topic. You can lift 711kg with a $$2800m^3$$ balloon with air at a temperature of $$100°C$$. Which means you balloon made of crude medieval materials and fuel can weight 500kg, you can still have a light 50kg daredevil with 100kg of molotov cocktails (or stones and arrows) aboard.

Even if your materials and fuel are really crappy, the amount of weight you have to lift grows like a surface, $$diameter^2$$. The force that will carry you grows like the volume of a sphere, $$diameter^3$$. This nice square-cube law plays in your favor. You can do estimations of the size your balloon needs to be before actually asking to your workers.

Interestingly, "science guy" only needs high-school level physics (law of ideal gases, Archimede's force, ...).

For the fuel (or the stuff you drop), note that distillation was known at the time (hence alcohol), and oil as well.

• Enough paper to make a hot-air balloon would be mindbogglingly expensive.
– Mark
Nov 19 '20 at 23:23
• @Mark not really its not paper by itself is is paper backing cloth.
– John
Nov 20 '20 at 4:01
• And the bigger it is, the bigger the target. Nov 20 '20 at 5:47
• @JustinThymetheSecond Target doesn't mean much bows cant shoot that high.
– John
Nov 20 '20 at 5:54
• @John 13th century, they certainly could. Balloons are notoriously unsteerable, so to have any accuracy they would have to be low to the ground. The rifles of the 14th century could take one out. The balloons would be 'siting ducks', or maybe a 'turkey shoot'. Nov 20 '20 at 14:44

Several other responses provide useful means for completely obliterating the typical siege targets (castles, walled cities, etc) and/or making them uninhabitable for a long time. This is not what a siege is meant to do.

Sieges usually are all about gaining control over places, not destroying them. A successful siege should render the place defenseless while causing as little damage as possible to both the structures and the inhabitants.

That said...

## Gold projectiles

This may sound like a joke, but assuming budget is not a problem...

Gold is certainly, by much, the densest material readily (for the very wealthy, that is) available at the time. It's almost 10x denser than your typical rock. That means, when launched from a trebuchet or catapult, it is able to carry almost 10x the kinetic energy than a piece of rock the same size.

Of course, gold is soft, which makes it a poor choice of material for piercing projectiles. Your science guy would probably know about full metal jacket ammo, and coating a golden core with something like steel shouldn't be hard for a skilled smith.

This should give you a huge attack boost against stone walls, a single impact might be enough to shatter a wall section. That is, provided you're able to reinforce your siege machine enough to generate the required power without breaking apart. Your guy and your lack of scarcity should be able to do this (having a better understanding of the load distribution, reinforcing critical parts with stronger materials like steel, etc)

As a bonus, you're already besieging the place you're throwing gold at, so it won't go anywhere. Your own victorious soldiers won't be able to sneak out carrying a half-ton ball of gold either. You should be able to get most of it back, once you've breached the walls and secured the city by conventional means (infantry storming the breach).

## Gold counterweight

Again, budget and density. According to Wikipedia (https://en.wikipedia.org/wiki/Trebuchet#Comparison_of_different_artillery_weapons), a 6000 kg counterweight gets you a 200 meter range for 100 kg projectiles. Using gold instead of stone gives you either more range, heavier projectiles, or both. Again, you'll need to reinforce the trebuchet for the additional load.

## Ballistics

Of course, medieval siege engineers already had their rules of thumb for estimating trajectories. However, your guy has a modern understanding of the physics involved in hurling things and watching them fall.

After a few experiments and some manual calculation and measuring, they will be able to produce rather precise data for every "if you put X more pounds on the counterweight, the Y pounds projectiles will fall Z steps further". This will result in far more accurate aiming and more reproducible shots, even when your guy is not there.

## Low explosives and detonators

Many available materials can be used to create a localized damage. Carbon, oil, alcohol, metals, even flour can deflagrate under the right conditions. Primitive gunpowder is definitely possible. Your guy knows all of this.

Your guy might also know how to ignite a spark by many different means. This means you can have impact detonators (two chemicals separated inside glass vials which break and allow them to mix), time detonators (strong acid inside something like a bladder which dissolves a hole after some time), fuses (a string soaked in something flammable)... whatever you need.

Instead of using a battering ram, your guy can devise a bomb that can be planted in front of the main door.

## Airborne infantry

Your guy will know about chutes, gliders, kites, etc. Of course, without modern materials you're really limited to start with. That doesn't mean you can't launch some soldiers from a modified trebuchet and instruct them to deploy some variation of a large sheet while they fall.

The thing will be barely survivable at best. However, after testing a few prototypes (hopefully not with human subjects, but this is a medieval setting after all), I'm pretty sure your guy can give them a better chance than being the topmost man in a siege ladder.

• I would think you would prefer to use lead, even if the gold isn't going anywhere because you don't have a lot of it. And I don't think it's true the gold wouldn't go anywhere. What's to stop people from chipping away at the gold boulder before you manage to recover it? After all, gold is soft. Nov 19 '20 at 22:33
• en.m.wikipedia.org/wiki/Operation_Bernhard en.m.wikipedia.org/wiki/Mansa_Musa At the very least you have succeeded in destroying the economy. Nov 20 '20 at 6:53
• Lead is a better solution as @DKNugyen said. If lead is not dense enough, go for Tungsten. Although it is still rare, W is hundreds of times (1.25CRC vs 0.004) more common than gold, harder, and just as dense (in fact, people used to use W to counterfeit gold bullion. Nov 20 '20 at 13:28
• You can refine wolframite using just chemical and mechanical processes, before anyone comes and reminds us that W cannot be smelted in the XIV century. Nov 20 '20 at 13:29
• "... launch some soldiers from a modified trebuchet..." The acceleration happens all within the first half second or so. The 'launch' would experience significant 'splat factor' G-forces almost as great as the landing. Also the 'First Law of the Trebuchet' - the rocks or whatever first have to be raised up to the level of the loadig platform. Then they fall back down. Then they have to be raised back up again. Then... see a pattern forning? The trick is not in getting something heavy, it is in getting something heavy HIGH enough, FAST enough for reload. Here is where cranes and levers come in. Nov 20 '20 at 15:25

Biological warfare--specifically, the black death.

Make some flea-resistant suits, send some soldiers to a place with the plague to gather rats and keep them safely contained. Add more rats gathered along the way to keep the population up. Bring a whole bunch of rats to the siege, infect them, attach streamers (you don't need to get fancy making true parachutes) wrap the rats up in their streamers (experiment to figure out how to wrap them to get the streamers to deploy properly) and trebuchet them into the city. The rats will gnaw off the streamers and now your city is flooded with plague.

If there is cholera anywhere around gather as much shit from it's victims as you can, lob it into the city also.

I would stay away from anthrax due to it's persistence.