A new way of invading a besieged castle?

Question

Forget those old-fashioned siege ladders. No-one wants boiling oil poured on them as they labour to the top. They need to get there much quicker and with no ladder to be pushed away when they are at their most vulnerable.

My commander wishes to catapult troops to the top of the castle walls. At the height of their trajectory the soldiers have no vertical movement so if this coincided with the top of the battlements they would land like a feather. Of course the machines would need to be reasonably close to the walls, but at night a ninja-like attack, immobilising one or two very surprised guards at a time, could be used to create a silent and undetected build-up of troops.

What, if anything, is wrong with this idea? How feasible is it with medieval machinery? How can the soldier's forward momentum best be cancelled?

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Note

For those who say it's risky; just consider how risky the usual method is.

Answer 1

Impractical

At the height of their trajectory the soldiers have no vertical movement so if this coincided with the top of the battlements they would land like a feather.

True, their vertical movement would be zero, but what about their horizontal movement? There are three types of arcs (roughly) if you want to talk ballistic weapons. You have a mortar-type arc, a cannon-type arc, and a catapult type arc.

Cannon-type arc is a mostly flat arc, maybe 15 degrees. This would give the soldiers far, far too much forward momentum and they would die as a result. Overall, a bad idea.

A catapult-type arc is around 45 degrees, but that means that the soldiers will have to travel as far forward as they do up, so if the castle is 50 feet high, they'll have to travel 50 feet forward. Also too fast - this isn't a person running fifty feet, this is a person going forward 50 feet using the same momentum that you use to fight off gravity for that extent. Also too much forward momentum. Remember, you're aiming for a thin strip of wall for your soldier to land on.

Then you have the last type of launcher, a mortar type which launches soldiers basically straight up. This is actually fairly simple using a trebuchet type mechanism and with little forward momentum, its conceivable the soldier would survive landing on the castle wall. The problem is that because you set up so close to the wall, the undefended trebuchet is now a sitting target for the castle defenders. And if you want to build structures to defend the trebuchet, you'd need a fairly large and complicated structure to protect it from the many, many angles of enemy attack and at that point, just build a siege tower, it's a lot easier and can hold more soldiers.

tl;dr; it'll kill people if its too far away and it's impractical to defend if its too close.

Answer 2

How feasible is it with medieval machinery?

Very feasible. Trebuchets were a thing in Europe around the 6th century AD and are very calibrable (yes, this is a word).

What, if anything, is wrong with this idea?

Murphy's Law, and not just how you imagined it when you read it.

Edward A. Murphy Jr. was working on project MX981 (a spiritual predecessor to Jackass) when he coined his famous law. The whole thing revolved around a very cranky rocket sled, meant to test the effects of acceleration and deceleration on stuff and PEOPLE.

His most famous volunteer, John Stapp (probably related to Ryan Dunn someway (also I just found out that he's my compatriot!)), seemingly enjoyed being used as a guinea pig on the rocket sled:

By June 8, 1951, a total of 74 human runs had been made on the decelerator, 19 with the subjects in the backward position, and 55 in the forward position. Stapp, one of the most frequent volunteers on the runs, sustained a fracture of his right wrist during the runs on two separate occasions, also broke ribs, lost fillings from his teeth and bleeding into his retinas that caused temporary vision loss; in one run he survived forces up to 38 g.

These things happen whether you are accelerating or decelerating very fast. But that was a rocket sled, which did it kinda gradually; Your human catapult will give its ammo all the impulse in a fraction of a second. All in all, you'd be squished inside if you attempted to fly like that.

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Even if you have an army of supersoldiers who can survive that, another problem arises: in order to have close to no horizontal speed, you'd need to have the catapult close to the wall - where it becomes an easy target for pit, tar, and fire. Not much different from using the stairs on this regard.

But if you position your catapult far from the wall, the soldiers will land with A LOT of horizontal speed. It would be like leaping from a car going very fast on a highway.

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Suppose you managed to solve the problems above: stairs are cheaper and have a better ratio of soldiers per second than catapults. Once you launch a soldier, it will take some time to ready a catapult for the next one. Even if you have lots of catapults, each soldier landing on the battlements would be easily surrounded and killed.

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And that's considering you don't miss. Surely, medieval engineers could hit far targets with some accuracy, but changing wind conditions, inaccurate measurements and blind bad luck would ruin the day of someone who went too high or too low.

By the way, once you calibrate a trebuchet for a certain target, you need to always use ammo with the same profile and weight to keep hitting it. Different soldiers with different weights introduce a problem here. Maybe you can solve this by making the lighter ones carry stones as they are loaded in the catapult. But different people will have different aerodynamics.

Answer 3

Well, it all depends on who you toss up over the wall, dunnit?

While your overall plan is sound, I think catapulting a fully armed soldier up onto the wall is a less than ideal solution. Logistics you see: you'd really need to get several up there, and then once on the wall, what are they going to do then? They'd either have to fight off the whole castle in order get down and unlock the front for your lads, or else would have to beat off hordes of castle guards while your folk set up & ascend their scramble ladders.

You'll want something small and light that can easily get up there and really astonish the defenders, so I'd take a page from British history:

Just catapult your world's version of the Rabbit of Caerbannog into that castle and within two minutes the front gate will come crashing open and the whole place will be emptied of defenders! A sort of shock troop, you see?

They'll be at your mercy and your lads can then begin the pillaging, sackage and rapine.

Answer 4

If the terrain allows for it, nearby elevations higher than the castle wall, you could hang-glide or parasail down to the wall tops. The creation of modern hang gliders or parasails is probably outside the limits of medieval technology, but the physics which make them work is still in affect.
If the terrain doesn't allow for a gliding approach, consider using hot air ballons.

Answer 5

Is there a reason why you don't want your troops to use one of the more common methods for reducing a fortified objective? These include: a siege tower, digging mines under the walls, using catapults or associated siege engines to batter the walls, or constructing a ramp over the moat up to the top of the wall so your men can charge across (as the Romans did at Masada).

And there are two very simple & often practiced tactics. One is finding a traitor inside the objective who will provide you access in return for money or other items. The other is simply surrounding the objective -- thus preventing food, supplies & water from reaching the defenders -- & starving them into submission.

Any of these would be far more effective & easier on your troops -- although much less imaginative -- than your plan.

Answer 6

The catapult method requires precise calculations, and the vertical velocity is not all you have to worry about (acceleration at startup is bearable): horizontal speed remains more or less constant during the whole trip, and can be significant.

For example, launching someone twenty meters upwards takes about two seconds. Launch speed vertical component is about 21 m/s. If the rampart is 50 meters away from the catapult, this horizontal distance needs to be covered in the same two seconds, giving a constant horizontal speed of 25 m/s (multiply by 2.2 to get it in mph, or by 3.6 to get kilometers per hour). Launch speed is then SQRT(2121+2525) = SQRT(1066) = 32 m/s, and is reached in about one third of a second. 0 to 32 m/s in 1/3 s is almost exactly 10 G. A big shock, no doubt, but totally survivable if you're properly prepped for launch.

Launching from ten meters away is more feasible - five meters per second are just 18 km per hour, or the speed you'd be falling at if you fell from 1.3 meters. Launch speed is lower. But at ten meters from the ramparts, the catapult is in boiling oil range.

The ninja needs something to stop their forward motion, and do that in very little space, but not too little because otherwise the shock will be the same of a hit against the wall. A sort of flying squirrel suit might do: curl into a ball when launched, then open wide arms and legs and (provided you're somehow oriented in the right direction - maybe use a trailing "tail"?) your cross-section trebles and your airspeed drops.

With excellent agility, a padded flapped wingsuit and lots of training I expect you'd be able to withstand an end-of-flight speed of 10 m/s, which means you can get at the top of the 20 m high wall from a catapult 20 m away (outside flaming oil range, but still in arrow, stone, and general mayhem range).

Answer 7

Giant Slingshot + Glider/Parachute

So the acceleration from a catapult/trebuchet would likely be dangerous for a human, however searching for "Human Slingshot" will yield a ton of slightly safer launch mechanisms (though most keep the payload secured for the duration). These rely on the contraction of an elastic material and so the acceleration will be more uniformly distributed rather then a sudden sharp spike at the start and therefore be much more survivable.

Upon arrival at the top of the wall there will still be considerable forward motion, and precision being extra important when you want a relatively injury free deposit atop the fortifications you'll want a method of arresting forward motion, guidance in case of inaccurate launch or interference from weather and ability to dodge incoming arrows or traps set upon the battlements. To that end a modified wingsuit should allow for fairly quick deceleration as well as some maneuverability to achieve the ideal landing spot. Ideally this wingsuit would have some sort of rapid stow/ditch mechanism as it might decrease the dexterity of the wearer in combat. These wingsuits would be a bit more akin to a glider/parachute than the modern wingsuits we see, but with sufficient volunteers and testing a happy medium could likely be achieved.

As for testing, I'd recommend a) volunteers who can swim, b) testing launch into a large pond, c) testing arresting-wing-suit via an elevated pool with plenty of padding.

Answer 8

You can stop forward momentum with a long rope. Tie one end of it round your ninja, and run the other end through something that applies friction, like looping it around several rough wooden posts. You can adjust the length of the rope in advance to ensure they are stopped at precisely the right place, and to control the strength and duration of deceleration. To keep the pull horizontal, lift the rope with a A-frame. Use a highwayman's hitch for a quick release. All you need is lots of rope.

Although if you just want to be able to climb ladders faster, a 'gin pole' hoist with a dozen mates on the end of a rope to lift you would probably be much easier and safer. I think that mechanism was used by Heron of Alexandria, so suitably ancient.

Answer 9

Horizontal Momentum & G forces are Solvable Problems

The key is in the OP's stipulation:

Of course the machines would need to be reasonably close to the walls

While most people think about catapults as being strictly trebuchets, there was another far more practical design of catapult for this problem: Ballistas. Unlike a trebuchet, it accelerates its load in a linear trajectory instead of an arc trajectory; so, it is much easier to fire a projectile at a predictable angle, and much easier on human biology.

Now for G force: your average castle wall is only 12m tall. This means you can roll your ballista right up to the castle wall and fire a person up at ~85 degrees at a velocity of ~8m/s. If your ballista has a 2m long draw length, this means that your person will experience ~4.2G* while being fired. Most people can withstand about 5Gs without a modern flight suit; so, your soldiers should be able to maintain consciousness when fired. Because of the high angle of fire, the soldier will be moving horizontally at just under 1m/s which is a bit slower than the average person walks; so, stopping your forward momentum when you get on top of the wall will be very easy.

*SEE: https://www.omnicalculator.com/physics/trajectory-projectile-motion

The Problems with this Design

While the technical limitations can be overcome, the reasons such a siege engine would not be popular are many:

1. It is much more expensive than a ladder. Ballistas required some pretty hard materials to make in bulk which is a big part of why they fell out of favor. Each catapult would need over 100ft of thick cordage made from human hair and animal tendons and hundreds of pounds of heavy metal fittings that would have been very expensive in the pre-industrial era.
2. It is harder to get to a wall than a ladder because it is so heavy. This means you may be less likely to die during the getting on the wall part of the siege, but more likely to get shot by archers in your approach to the wall.
3. Reload time & complexity. Large ballistas would take a minimum of 20 seconds to reload under normal circumstances, but this device also has to be calibrated to a specific person's weight. If your first soldier weighs 90kg, and you next guy weighs 70kg, then your second guy will overshoot the wall unless you adjust the torsion springs first which would extend the reload time to at least a minute. This problem can be partially overcome by organizing your squads into weight classes, but even solving for the need to adjust the springs, you will still not be able to get as many people on a wall as quickly as with a ladder.
4. It is not actually quite. Large catapults (both trebuchets and ballistas) make loud and distinctive sounds when they fire. So if your goal is a quiet night raid, you will not be able to stealthily get a lot of guys onto the wall before someone notices your presence.

Consider instead, just fixing the problems with siege ladders

One solution is ofcourse siege towers, but again, those are really expensive and harder to cross uneven terrain with than ladders; so, they still don't entirely replace siege ladders. But, what if you just make ladders that can't be pushed over? Basically, there are two ways you can push a siege ladder over, one is to push it backwards, and the other is to push it sideways.

Without adding much weight to a ladder, you can add a few extra bits that will prevent it from being pushed over. To prevent your ladder from being pushed over backwards, just add a nice big foot to it. you can bring your ladders to the walls just like normal, stand it up, spin the foot into position and lock it, then bring the ladder against the wall. The means that the feet of your ladder are no longer a fulcrum that it will just pivot over, now the defenders are at a huge mechanical disadvantage trying to push the ladder away, and once you put the weight of 1 person on the ladder, leverage will favor you so heavily, that it would take many men to even get the ladder to budge at that point.

Pushing a the ladder over sideways is also going to be very hard if the defender has a crenulated wall because they first have to push your ladder backwards enough to get around their own battlements, but just in case, you can also add extra bits that extend between the crenulations so that the defender's own walls prevent them from pushing your ladder sideways.

As for boiling oil and arrows, this is not as big of an issue with siege ladders as you may think. As your own illustration shows, a shield gives great coverage from any attacks coming from above. Also, the exact angle of a ladder typically puts you in the blind spot between where defending archers can shoot at you from over the battlements or down through the machiliations.

Answer 10

There is a way

I read @The Square-Cube Law and @Halfthawed and I mainly agree with them.
There is a way this might work though. It would be extremely risky for the troops involved but not exactly suicidal. I don't know if it would be compatible with your story and this answer may be too much story based. Up to you to decide.

Preconditions. the besiegers have:
a group (20/30) athletic, motivated and well trained soldiers
several trabuchets that are strong enough to launch the soldiers over the walls and into the town
previously have bombarded every night the town launching inside corpses of rotten goats and sheeps with the apparent motivation of causing pestilence
have done extremely accurate survey of the landing area and spotted a suitable one (most likely with inside help)
the landing area is a slanted roof. It's slope is just at the right angle that an incoming soldier would not likely be injured. The soldier will slide on it and probably fall down in an inner courtyard
the inner courtyard is owned by the aforementioned inside helping citizen(s). A large, tall heap of hay has been prepared to dampen the fall of the soldiers. together with the soldiers some launches may be of bags with armors and weapons, dampened in wool sacks
the launches will go on, as usual, all night long. Some trebuchets, aimed at different parts of the town will throw in dead goats as usual

the group of soldiers will then be gathered in the inner courtyard during the night. They will put on their armors (likely light ones) and just before the break of dawn do a surprise attack at the closest town gate that has been identified as the most likely to fall.
Outside troops will have silently gathered at the convened point and be ready to storm forward at the signal once the gate is open with the purpose of holding control of the gate for the rest of the army.
the main army stays on its usual positions during the night to avoid alarming the defenders. As soon as the gate is open they move so they can storm the gate.
Smaller forces in other parts of the walls may cause noises and movement to confuse the defenders at least for the first crucial minutes.

Ideally all this should be staged a long time before the siege to measure the distances and angles right without raising suspicions and buy the most ideal property with a suitable roof and courtyard. Once bought the roof could be 'fixed' to be of ideal angle if needed.

Note: The inside helping citizens may not be strong enough (elderly? women? unarmed?) to overcome the guards of course. Otherwise nothing of this would be necessary.

P.S. note that the farthest shot of a man from a cannon is just 59 meters. The target area needs to be pretty close to the walls.

Answer 11

Use a Pendulum

Instead of catapulting someone from low ground on a downwards arc, create a very large A frame swingset. It will need to be a bit taller than the walls and should be wheeled into place right before the assault. Have several men stand on individual swing boards at the bottom and have a hoist rig bring them back. Let go of them at different times so it's a harder shot for archers on the wall. If you have the distances calculated correctly, they'll arrive at the apex of their swing at the top of the wall and they'll just need to step off to begin the assault.

Bonus physics: They can carry weights that they drop at the bottom of their swing which will mean you have to hoist them less distance backwards to reach the same height of the walls.

     O
    \|       /~~~~~~~~~/\
__| /\ ~~~~~/         /  \
|                    /    \
|                   /      \
|                  /        \
|                 /          \
|                /            \
|               /              \
|              /                \
|             /__________________\
             OOO                OOO 

Answer 12

Outfit your troops with inflatable suits, and find some high ground. A catapult throws objects with a force of 89.0 kN, which is more than enough to send a person flying far. As the suit inflates, it will provide a larger surface area, reducing the pressure on the wearer, and giving some room to buffer out the sudden acceleration. And, it will help with the landing by increasing surface area to increase drag and reducing density to increase buoyancy (which will really help if you end up landing in water). As a bonus, these inflatable suits will get your airborne soldiers pushed further, so if the catapults are positioned correctly, the wind can do a lot of work on getting troops to their destination.

Answer 13

The biggest issue with this is conservation of energy. When you pull back the catapult, you are storing energy. The catapult converts this potential energy into the kinetic energy of the unfortunate invader. While the vertical energy is zero at the peak of the arc, the horizontal energy is not, unless they were fired exactly straight up. This is naturally impossible. But never fear! There are two ways to do this with sufficiently brave/stupid soldiers and enough math.

Method 1, very close catapult.

if the catapult is very close to the wall, and the angle of fire is very low, there should low enough horizontal force that the soldier will not fall of the wall to his death. However, you would have to get almost the perfect height, because if the arc is to low, the soldier will smack the wall. if the arc is too high, the soldier will fall to far before hitting the wall, and all that kinetic energy will go straight into the soldier's ankles, shattering or injuring them. Or, the defenders could dump boiling oil on your catapult, because it would essentially have to be touching the wall.

Method 2, Surprise flying kick.

This requires a lot of energy-absorbent material at the top of the wall to work, but if you calculated it so that the point where they only have vertical motion is right when they hit the material, the material would (hopefully safely) bring them to a stop. This material could be something like a guardhouse bed. You could also use a group of soldiers as your energy-absorbent, hence, the surprise flying kick. You also might want to strap cushions to the soldier to reduce the very real chance of them dying on impact.

Conclusion

This method would require lots of very accurate math to pull off, and it would probably go wrong anyway. However, It might work, and no other strategy in the history of siege warfare would have the same level of awesome. Final note, it would take a long time to get a full scale invasion force inside the wall. Cool idea!

Answer 14

Add parachutes and it will work.

Parachutes are very low tech - stone age in fact, requiring nothing more than cloth and light ropes. It's amazing humans didn't invent them earlier - we've had cliffs for billions of years and people who want to get from the top to the bottom in a hurry.

Trebuchet the warrior, who maintains a tucked position to more predictable trajectory, far past the walls. The paratrooper deploys his parachute when he's roughly over a suitable landing spot.

Set the trebuchets up just outside the range of a bow. Not too far back - you don't want the warrior to faint due to excessive launch acceleration.

Answer 15

Catapult ramps instead of people?

Instead of catapulting men up, would a catapult work for getting a ramp into place quickly? One end fixed, hinged, some distance from the castle, catapult the far end to flip it into place.

Would have a couple of advantages over siege ladders in that the weight and angle would make it harder to push away from the wall, it's shallow so pouring oil down doesn't work quite as well, and might be wide enough to allow more than one soldier attack together.

Not sure the physics will work though, as it might be difficult to build a ramp that doesn't splinter from the speed it hits the wall, which isn't too heavy for the process to work in the first place.

But if it did work, firing a few ramps up simultaneously along the wall might disrupt the defenders while more of your troops attack with standard siege ladders.

Answer 16

You need to constantly shoot rocks at the enemy walls for a few days before, so they don't get suspicious when you start the sneak attack. A super tall enemy flagpole on a tower can be abused to stop the frontal movement through some sort of bungee jumping mechanism. A weight at the end of an elastic rope used by super quick hands might somehow get a few of your smallest, strongest athletes up there. Each of them would carry a part of a gigantic net with them. They'd install the net to make it easier to land. Then you can shoot more heavily armed soldiers into the net with another, bigger counterweight trebuchet. The launched soldiers would have to wear weights or less armor depending on each of their bodies weight. The percentage of the soldiers that survive the impact without heavy injuries might want to spread immediately, making it harder for the defenders to surround their positions on the wall. The ones that arrive a little crippled, will drink a lot of rum and use the completely crippled or dead ones as cover while protecting the net.

Maybe you should make your non-crippled soldiers shout: 'Flee, the wall has been breached. Flee!' This might make it harder for the defenders to organize their troops and find the place where the enemies are coming from.

Then you can abuse the chaos and attack the walls everywhere at once with long ladders, like normal people do.

Not situational at all. 10% of the time it works 100% of the time.

Answer 17

I'm going to explain how somebody might attempt to get launched from a safe distance by a trebuchet and then land with very little acceleration on a wall.

Use two gigantic trebuchets with one projectile each: One is a rock, one is a soldier. The trebuchet launching the rock is about 70 meters closer to the wall.

The projectiles are connected through a light and strong rope with a length of let's say 200 m.

The rock is accelerated much more and has a steeper trajectory than the soldier.

At the start the soldier holds on to the rope and 'slowly' lets go of it, as the trajectory between him and the rock diverge more and more from each other. (good gloves are recommended)

This way the soldier will get more speed that doesn't have to be gained over the short distance the trebuchet has to accelerate him, which makes it possible to launch him much further without killing him right at the start through g-force. Instead of about 20 m/s , he'd get e.g. 36 m/s and gain e.g. 60 meters in height, which would make it possible to launch him almost 200 m instead of only 50 m.

Now accelerating isn't a problem any longer. Decelerating is.

Since the rock has a much steeper trajectory, the soldier will be able to slow down his forward movement by holding onto the rope and or pulling himself towards the rock. He has to to do this rather quickly.

If the rock is launched at an angle of 88 degrees with 50 m/s, which is really generous, the soldier will have only a little more than 5 seconds to decelarate by pulling himself towards the rock before it certainly would be lower than the destination point, which would make it useless for decelerating vertical velocity.

Depending on the exact distances the trebuchets have from each other and from the wall and on how effective the technique of pulling oneself towards the rock, the weight difference between the rock and the soldier would vary heavily.

If everything is calculated perfectly, the soldier loses all his velocity by the time he reaches the top of the wall.

Your soldiers will have to practice a lot getting launched into a lake, before attempting to land on a wall.

Answer 18

Shortest Answer: No. Physics works according to component forces. So they would hit vertically like a feather, but the velocity needed to get them to the wall while being out of firing range would be lethal.

Think about how far you can throw a stone horizontally. If you hit me in the face with the same hand that throws that stone I'd be unconscious. What OP is talking about here is significantly more force than that, as it has to motivate a whole human being from a single starting impact, yet displace him further than a stone can be thrown by hand. Anyone flung by such a catapult is going to have tremendous horizontal momentum by the time they arrive at the apex of their parabolic arc.

Answer 19

A "launch" with a short acceleration point would act a lot like jumping off the wall onto your feet; it would kill you. The acceleration region has to be extended long enough not to kill the human.

Physics is reversible, up to entropy.

Horizontal velocity at the top of the wall would kill the human, so you have to make a near-vertical launch.

So we have a device that gently accelerates a human being upwards with little lateral velocity and stops at the top of a wall.

We call this a ladder. It even runs on the human's muscle power!

A siege tower with stairs also works.

An elevator is probably way too complex.

Answer 20

You seem to forget the archers that defend the castles.

There is already a load of strong guys with strong (long) bows and armor piercing arrows ready to shoot at anything that shows themselves as a target.
When your soldiers try to land on or in the castle they have to go slow speed and will be outlined against the sky. Perfect for a 'who can place the most arrows' competition, or a 'who can place the arrows where they do the most good' competition.

Any armor that will be too difficult for those arrows will slow your soldiers down so much that the defenders have an easy time to take them down on arrival.

The defender archers do not need to be visible from the ground, they can even be much lower down in the castle as long as they have a good knowledge of where the invaders come from. (Just send out a lot of arrows, not easy to avoid those.)

Answer 21

The biggest challenges to your idea are economic

A specialized siege engine is going to be very expensive. Regardless of what technology it uses, the forces involved mean it will need several large, finely crafted parts made of strong materials. It will require skilled operators, who know how to use it without the forces tearing the machine apart. Such a machine will cost a lot to make and use, which means it needs to do a big job, like knocking holes in a castle wall to be worth building. The trouble with your idea is that getting a few men to the top of a wall can be done more easily with much cheaper equipment.

Unlike a siege engine, siege ladders are very cheap. Ladders are a prehistoric technology, and while siege ladders are a little heavier than normal ladders used in civilian life and have a few added bits to help anchor them to the ground and the wall, any carpenter should be able to build one. Wood to build them from is available in every forest the army walks past and they don't require particularly fine craftsmanship (though taller walls do require longer, moderately higher quality ladders). Ladders don't need many men to operate either. You probably only need a half dozen men to carry and raise each ladder, and for shorter ladders you may be able to get by with fewer. Together with the low cost, that means you can have a lot of ladders. While your man-throwing engine might theoretically be "better" in some way than a single ladder, it's going to have to be able to compete against dozens of ladders to come out ahead on cost effectiveness. An escalade (which is the technical term for an assault on a fortification by ladders) is usually a mass attack, used when the attackers outnumber the defenders quite heavily. It would be implausibly expensive to build enough of your man-throwing engines to take a castle by storm.

On the other hand, if you just want to enable a stealth attack, your man-throwing engine also suffers against a simpler and cheaper technology: A rope with a grappling hook on the end. It is a whole lot easier to get a coil of rope up close to the walls without any enemies noticing. A rope can be carried and thrown by the very same man who's going to climb it without needing any help from anyone else (the rest of the army can be mounting a diversionary attack elsewhere, if you want). And obviously, a rope is very cheap and very quiet in operation. Ladders might also be useful for some kinds of sneak attacks (where you don't need the attack to remain secret for very long), since they can be relatively quick and quiet to deploy!