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For the sake of simplicity, we will use 1200's Medieval Paris as an example. The city has walls 40 feet high and the initial defending army (army A) is 1500 strong. The attacking army has to breach the city and storm it. The citizen militia in the city equate to another 800 ill-equipped defenders.

It HAS TO BE TAKEN BY STORM! My attacking army (army B) has tried breaching the catacombs running under the city but the elite knights of Army A are stationed there and they are unable to advance. They tried bribery but the last umpteen attempts by treacherous Army A soldiers have been caught and the traitors promptly killed.

How do many men do I need to breach the walls by force?

EDIT: Army B has 3 trebuchets, 10 onagers, 2 battering rams, and a team of sappers as well as a single siege tower set up.

They have 1000 scaling ladders (the defenders are prone to catching them on fire) so they have many in reserve.

A large relief force of 2500 knights is approaching and will arrive in two or three days. After breaching the city the attackers must hold it indefinitely. It has ample food stocked away (another reason a siege wouldn't work)

The following map is a general overview of the battle:

enter image description here

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  • $\begingroup$ What are the weapons of the aggressors? Do they have battering rams? Trebuchets? Sappers? Also, your question title is different from that asked in the question body. $\endgroup$ – Frostfyre Apr 20 '15 at 16:30
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    $\begingroup$ This question is way too broad - even when you specify "by force", there are a million different strategies that can reduce the amount of men required. Furthermore, without a detailed map of the scene of battle or a detailed description of exactly what I have available to me and what the defenders have available, it'll be near impossible to formulate an optimal strategy for storming this city. $\endgroup$ – Aify Apr 20 '15 at 16:38
  • $\begingroup$ @Frostfyre They are essentially the same question. Not word for word, but they mean the same things. The attackers have three trebuchets, 2 rams and a group of sappers. They also have a single siege tower constructed on site. $\endgroup$ – JDSweetBeat Apr 20 '15 at 16:42
  • $\begingroup$ @Aify I am working on a very basic map of the battle. $\endgroup$ – JDSweetBeat Apr 20 '15 at 16:45
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    $\begingroup$ You have sappers. Just protect them roman turtle style as well as with cover fire, have them blow up the front gates. And rush in. Thats the concept of 'storming' in $\endgroup$ – Spacemonkey Apr 20 '15 at 16:49
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I think this sounds like a multi-step question:

  • Step 1: Surround the city with your forces to prevent reinforcement or escape. If you expect a relief force, you need double walls to keep the besieged army in and the relief army out (circumvallation and contravallation).

  • Step 2: Create a breach in the walls.

  • Step 3: Actually storm the city through the breach.

  • Step 4: Hold the city down after the initial storming.

Step 1 will require a large force, much larger than the 1,500 defenders.

For Step 2, the siege engines are all helpful, but much depends on the size of the trebuchets, the thickness and quality of the walls, the countermeasures of the defenders, and so on. A week or more, perhaps much more.

  • Do the defenders have trebuchets on the turrets of their city wall? If so, the extra height should give them a nice range advantage. Setting up siege engines will be a long and costly business.
  • Unless the attackers are able to build larger trebuchets than the defenders, that is. Then they might be able to fire from impunity unless the defenders sally. That's why you can't use all the attackers to haul logs or stones for the trebuchets.
  • It might be most efficient to dig a tunnel (a "mine") under a turret or the wall and to light a fire to topple it. This will take weeks or months, but at least the miners won't be under fire from the walls. There might be countermines, however.
  • Battering rams only work if they can be roofed to prevent fire from the walls, or if archers can suppress the walls. The latter is mostly cinematic.

While the attackers try to create that breach, the defenders can tell where the attack will be coming. (Unless the attackers create multiple breaches to keep the defenders guessing.) So they can prepare. That brings us to Step 3:

  • The attackers are climbing up a rubble-strewn slope, and through a narrow opening. The first heroic knight can expect a crossbow bolt. And the second.
  • But sooner or later one of the storming party will live long enough so that the man behind him can reach a defender. One down, 1,499 to go.

A slightly later era got us the terms forlorn hope and practicable breach. Firearms would make it even more bloody, but the principle holds.

Historically, the result of the storming would be a sack (again a slightly later example).

Afterwards, you have to hold the city down. The numbers for that depend on the civilian population and their attitude.

TL;DR if trickery failed and you have just three days, you can forget about the trebuchets and rams. Those thousand ladders and a real lot of casualties are the way to go.

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    $\begingroup$ I feel like you,d run out of time. It can take a easily a couple of hours just to surround the city, let alone set up shop to effectively hold everyone in from your positions. I can take quite a while to 'breach' a city by taking down the walls, we are talking about days. Lets use handwavium and do as if you had all necessary materials close by and all the man power needed to use it, I'd still be thinking a day. Let's cut it down to half. Now you need to storm the city. Depending on the size, again days. Then you have the castle.... then you need to defend. (Plug that breach etc...) :( $\endgroup$ – Spacemonkey Apr 20 '15 at 19:16
  • $\begingroup$ So like you said, trickery or lots of bodies. :( $\endgroup$ – Spacemonkey Apr 20 '15 at 19:17
  • $\begingroup$ That's a good analysis, but unfortunately it fails to answer the primary question "What is the minimum number of medieval troops required"? $\endgroup$ – Philipp Jan 6 '16 at 18:45
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Based on the current information I am seeing, army B may be able to take the city and hold it with about 1000 men.

Note that at current time of answer, op has requested that defenders are spaced evenly along the wall, with ~100 men at each gate. This tells me that there is limited space on the walls, and it's more effective to rush a single point than to try and fight all the defending soldiers at once.

With siege engines, which can take down castle walls easily, all you have to do is focus fire. This allows for multiple strategic responses.

Assuming worst case scenario where in 2 days, you'll be crushed by 2500 knights, you need to capture the castle within a day and a half so that you have at least a half day to prepare for the oncoming enemies.

Step one: Focus fire

You have trebuchets and onagers - use them. Other than 2 onagers, focus all projectiles on one point of the wall. By doing so, you open up several scenarios. With the other 2 onagers, put one on each gate and task it to block the gate with boulders so that no one can escape from the castle.

Important thing to remember: the elite troops of Army A are in the catacombs! They're not on the walls! Optimally, we're going to assume that about 500 of them are elite soldiers and as such, only 1000 other troops are left on the walls.

1) Everyone on the defenders side goes to the wall to fix it as it gets destroyed

Your response: Now that everyone has left the opposite side of the castle, your siege tower (or ladders, whichever happens to be stealthier for you) is free to roll up to the wall. Send up 800 of the 1000 soldiers (Pick the most well trained) you have in your army. Your soldiers are now on the wall, and have free reign against archers (which would likely consist of most of the defenders on the wall, since archers are the most useful on walls/in towers (which doesn't seem like the city has any)) and other soldiers. Because most of the enemy will be trying to save the wall your machines are smacking away at, you'll effectively be able to cut them down from behind. I predict that 800 soldiers should be enough for this job simply because a) they won't be expecting it - demoralization is likely, as well as having their backs to you being a massive advantage. b) the numbers advantage is much less in a narrow corridor such as on a wall. As soon as that happens, it's likely that the enemy will drop what they're doing on the wall, and you can stop firing rocks. This is to allow your task force of 200 armored knights to storm inside and take out the 800 ill equipped defenders (assuming ill equipped means badly trained peasants with pitchforks/knives, etc). If the wall hasn't crumbled by this time, you have 2 options: You can either use the battering ram to get through the rest of the wall, or the sappers. This will be possible because everyone is busy fighting your 800 men on the walls.

2) Everyone on the defenders side runs away from the rocks to save themselves

Your response: Split your forces into 750 and 250 men. Stop firing your siege engines. Send 750 men up the side of the wall where the entire enemy force has congregated, and send 250 up the empty side (as quietly as you can). Use the battering rams and the siege tower as cover (and a fat distraction) - stall and distract the enemy force while your 250 men dispatches the enemy from the back. Then, you can take out the militia easily.

3) They don't do anything, the wall falls within an hour, they take massive casualties from the rocks hitting them while they stand on the walls, and you march your men through the wall.

The followup: Marching 1000 men through the wall can easily take out the 800 militia inside the city. At the same time, the rest of the troops have to get off the walls to fight you - archers can't fire into their own city for fear of killing their own people, soldiers on walls are out of range. This means there are now trickles of soldiers coming into the city from the walls, that you can cut down easily with some choke points in the city streets.

Up till this point we've only covered what to do to storm the castle. Now, to hold it:

OP has mentioned that he has sappers, and that there are catacombs below the city. What needs to be done now to hold the city consist of several things:

1) Seal off the catacombs. You don't want those "elite troops" to come up and cut you down. Dispatch some sappers and soldiers to seal off all entrances/exits of the catacombs. They'll starve/drown/suffocate to death down there. In fact, if at all possible during the siege process, you will want to do this step ASAP.

2) Fix the wall (if needed). If you put a hole in the wall, use your sappers to fit it. Better yet, booby trap it! Reinforce it if you can, you can take apart the siege tower/rams and use its parts to do so if necessary.

4) Set up the onagers on the walls, and the trebuchets in the city. Use them to attack enemy siege engines should they bring any, or to wreck general havoc on advancing forces.

3) Send a fast moving scout (or something like that) to your home country asking for reinforcements. This is almost the only way for you to survive indefinitely. When your help arrives, you can commence a pincer attack on the 2500 relief troops. Otherwise, you're in for a long wait until the relief troops leave....

Disclaimer: the above is a very rough strategy based on optimal conditions and responses - without more detail concerning the lays of the land, any obstacles, or troop count/types of the enemy, I cannot devise a more elaborate strategy. One of the main reasons this plan should work is because we know the status of the relief troops. Under normal circumstances, where we don't know if there will be relief troops, then the strategy would certainly involve many more people, such as in O.M's answer. However, since we already know it will take at least 2 days, we don't have to worry about it and can focus solely on taking the castle as fast as possible. While technically you should be able to hold the city for a while with the remaining surviving troops (probably about 600 if you got lucky) This is a bad guideline and you should probably send double or triple the amount of troops to guarantee success. Murphy's law: Anything that can go wrong, will go wrong...

EDIT: After some discussion with friends, a second strategy requiring about 1200 men, but less distraction dependent

Position the trebuchets such that the projectiles roll across the top of the gate area/battlements.

Instead of aiming for the walls, aim for the people on top. This accomplishes several things: 1) takes out troops on top of the wall 2) blocks off reinforcements to sections of wall (if your boulders stay on the wall).

In 1200 century, battering rams had roofs. This means you can send rams and sappers to the front gate to take it down quickly. The roofs also protect (a bit) from falling debris as your trebuchets wreck havoc on the gate area. Onagers can be used as a distraction, and as a terror tactic, lobbing rocks into the castle innards, hopefully taking out some of the militants.

As soon as you break through the gates, you have free reign on the militants, and as stated before, chokepoints can quickly dispatch any incoming trained soldiers as they leave the walls to defend their castle. Remember to send sappers out to seal the catacombs!

The 200 extra men are required b/c you're probably going to lose people moving the rams into place, and a frontal attack means you're likely to lose more soldiers since you don't have the element of surprise.

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    $\begingroup$ I'd say the focus idea is quite good. However, seige engines do not take down walls quickly except in Hollywood movies. Overwhelm with many ladders instead. $\endgroup$ – Dronz Apr 20 '15 at 22:24
  • $\begingroup$ @Dronz siege engines can take down walls quickly, dependent on the thickness, quality and material of said walls (as well as slope and other factors). In fact, the whole point of trebuchets were to take down walls, and seeing as OP has noted that he has several at his disposal, I think it's reasonable to assume that that wall is going down in a couple hours or less. I'm pretty sure even the toughest walls have trouble defending against 200-300 pound rocks. (Also, the fundamental problem with overwhelming via ladders is that it increases men required) $\endgroup$ – Aify Apr 20 '15 at 22:31
  • $\begingroup$ I disagree. Accuracy and rate of fire are not great, and these are serious walls that may take damage, but are not going to be reduced to something you can send an army through even in three days' time. If trebuchets were that effective against castles, no one would bother spending years to build a castle. $\endgroup$ – Dronz Apr 20 '15 at 23:00
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    $\begingroup$ @Dronz on the contrary, trebuchets were so accurate that once you got it to hit the correct spot once, you could reliably hit the same spot forever - great for taking down a specific section of wall. BTW, you may also have noticed that nobody builds castles anymore. Also, while trebuchets were in fact extremely effective, they were also completely stationary, and as such made massive easy targets for defenders to rush out and dismantle - usually, a trebuchet is more trouble than it's worth. But in this situation, they do their job perfectly, especially if enemy troops don't engage on you. $\endgroup$ – Aify Apr 20 '15 at 23:06
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    $\begingroup$ I grant that once you find your range, you may be accurate again. However, castles and trebuchets coexisted for centuries. Castles continued to be built (albeit with varied style) even after cannon became effective. Take the siege of Acre in 1191. 11 trebuchets (not 3) were used, and did eventually reduce the walls, but it took about a month for the attack to succeed. $\endgroup$ – Dronz Apr 20 '15 at 23:35
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Two to three days isn't long enough to breach the walls using the available technology, and is nowhere near long enough to siege the defenders out unless you can poison/destroy their water supply. Further, the need to hold the city against the relief force means either of these options will cause problems later.

This means your only option is to take the walls by storm. Given the limited siege train you've got (only one siege tower), most of your men will be going up via scaling ladders (and mostly coming right back down as the defenders push the ladders over). You'll also want to send the battering rams against the gates to force the enemy to divide their forces, but this is purely a feint: actually breaching the gates would leave the relief force with an easy way in.

Since you'll need to hold the city against the relief force, you can't get much use out of the artillery: because of the risk of hitting your own men, their only use in an assault is throwing incendiary projectiles into the city to further divide the defenders, and that rather limits your ability to hold the city afterwards.

A good rule of thumb when fighting in such unfavorable circumstances is that you need at least a five-to-one advantage in numbers, meaning a minimum of 7500 attackers. With this many soldiers, a second option for resolving the situation presents itself: leave a portion of the attacking army (around 1500) to keep the defenders penned up in the city, and march the rest out to defeat the relief force, as a two-to-one advantage is usually sufficient in the open field. Once that's done, you can settle in for a proper siege.

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The minimum number is hard to say, but here's what I'd suggest.

  • Note: three days is not long enough to breach the walls using siege engines, at all.

  • Even if you had a way to destroy the walls, you'd prefer to take the city intact, and use those intact defenses against the relief force.

What you want to do is gain the city before the relief force arrives, so your force needs to be strong enough to beat them once you are inside the city, and it needs to be strong enough so that your survivors from that battle will be enough to defeat the relief force while you are in the city. Use whatever size/strength estimates you trust to gauge the size needed, but probably at least twice the size of the relief force, or 5000 good men.

The most efficient way I can see to do this by force, is to take advantage of the size of the city, to split their forces, and then sneak in enough men to defeat them in battle in the streets.

Step 1: Scout, harass, and begin what appears to be an assault force concentrated near the northwest gate. Siege engines bombard that gate. Have scouts and skirmishers venturing all around the city, to appear that you are active in all directions.

enter image description here

Step 2: Start several diversionary feint attacks at night, at the northeast gate, and along the south west part of the city walls. Gauge the wind, and create smoke to spread through town, to reduce their situational awareness. Some groups have men using ladders to try to gain the walls. Have other men try to get into the gatehouses to open gates. Have a feint attack in the tunnels you mentioned. These are all diversionary attacks, however, not attacks in strength, but there should be adequate men in reserve so that if any of them meet with good fortune, you can send in a good-sized force through any means you manage to gain this way. The point is that in order to prevent you from getting a strong force into the city this way, and to respond, the defenders will need to spread their forces all around the city, everywhere but where you will concentrate your main force, which brings us to:

Step 3: After the diversion has attracted a lot of attention to moving groups spread out and not under their direct central command, the main attack uses boats and ladders and pours through the river opening coming downstream from the east. Advance the siege tower from Ab. St. Victor and have archers in it lend support to the river invasion. Secure "beachheads" as quickly as possible on the north and south bank and on the island with the bridges. Get as many men on the streets as fast as possible, and then defeat the enemy in general melee in town.

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Unless you have a cannon and you wouldn't for another 100 years then your only hope to take the city with walls that high and their own would be to either...

A) Leave a small force to hold the area around the city and a1) Trick them into believing you have left to attack their reinforcements and then counter-attack them when they attack your men.

OR

a2) Trick them into believing you have left and are waiting for them to come out so you can attack them when actually you are going to attack their reinforcemnts.

The second option is B and much more sensible.

B) Concentrate your artillery on a specific point on their walls and open up a weak spot for your men to attack. Do it quickly, you only have 2 days at most. Expect the reinforcements to beat your expected time. Focus all your efforts on their best men and kill as many of their elite knights as possible.

The answer to your question is at least 7000 men before you even consider staying overnight. No logical commander would even think to attack a city without a three to one advantage especially when that advantage is trimmed within 3 days. With only 7000 men you will not be able to build a wall around such a large city like 1200 AD Paris because if you build your wall too close to the city their men will attack your men. Even if you had 7000 men against a city like Paris you would not attack it with such a large relief force expected within 3 days. Personally I would mix things up by entraping the knights in the catacombs, maybe paying off a local to figure where they enter and destroy the entrance once their inside. Then I would use my artillery to burn the city, then attack the walls. If the cities defenders have any type of artillery I would abandon the assault.

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  • $\begingroup$ I got the question by watching the Joan of Arc movie (one of the countless) and seeing how she attacked Paris with a relatively small force and no siege equipment and got slaughtered. $\endgroup$ – JDSweetBeat Apr 21 '15 at 12:49
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Interesting...with the norm being to avoid pitched battles, maybe most pitched battles were a result of a withdrawn siege, if 2500 knights are coming to relieve the defending party, then you ultimately have two paths, take the city before the relief force or withdraw but risk both forces now joining which im sure they would pursue to eventually besiege them in turn which gives the initial attackers time to build up a bigger army...other than the brovado and kingly pitched battles, maybe most pitched battles were an escilated with the back and forth castle sieges, food for thought. For the question i wouldnt say theirs a particular set number of men needed to *hold a city but rather how many are available, since it seems western european warfare takes on a byzantine reserve army strategy rather then the old roman frontier, so id guess about 1/3rd of available troops is enough, as holding cities and castles are temporary at least in the short time following, but after months or years, the numbers used to keep order would, rise and fall according to an infinite amount of factors of possibility and time, their just isnt a set number to conquor a city or fotress, it could be 1500 defenders, and you have 300 yet with cunning maybe a false letter tells of a force coming from a nearby castle with 3000 men, and then the defenders surrender, variables.

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