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My kingdom has 2 entrances in a hilly field by the ocean shore, comprising over 100 acres. The entrances are the size of a road and can carry carts up and down tunnels leading 12 stories underground to a large city. The city has thousands of people housed in caverns, and this city supports an army of 400-500.

Th underground city owns a small portion of land on the surface, only 100 acres, or about 75 football fields. It is a dense orchard of apple trees and it is in a hilly environment like the picture below.

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The underground city is being attacked by a kingdom, who are Viking-like with 800-900 men. These raiders land on the shoreline and ambush the entryways in the night.

I would like ideas on how to use this hilly environment with technology available in the Middle Ages to take back the land on the surface from the Viking people for the underground kingdom.

The problem I face is developing a good plan for victory when the underground people have been ambushed and forced to close the doors to the main entrance sealing themselves inside. They are not going to die down there but they cannot stay there forever, so how do they take back the surface? They cannot simply wait it out because if the Vikings control the entryways, then they are stuck down there forever. The Vikings will also not stop their siege because they want the riches of the underground city.

So the idea is that neither of them have any backup or help in the war, it is one kingdom against the other for this first battle, with the outcome needing to be the underground claiming victory. Poison is an option, alchemy, like real alchemy used in the past is used in this world. So more or less if chemistry is involved I'm fine with that as well, seeing that they're similar. The idea was for them to be outnumbered, that is why I was looking for other methods to "even the battlefield" but I don't want one to be overly powerful and downright kill the other. But smokescreens, poison or anything else you can do with chemistry or old middle ages technology that could help them win, I'm open to suggestions.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch - Reinstate Monica Aug 27 '18 at 5:44
  • $\begingroup$ If it's a big city, it would be very unlikely to have only two entrances, there would probably be a secret entrance somewhere, e.g. used by smugglers, secret service, army, or in case of a natural disaster, e.g. flood, earthquake, fire. Having only two entrances is a great danger even in times of peace. Ask yourself how hard it would be to create a new entrance? $\endgroup$ – Sulthan Aug 27 '18 at 10:39
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So Technically, your city is at a huge disadvantage being underground and only having two entrances. I'm going to assume you've closed both your entrances, and that they are super heavily defended via traps or just giant gates so that the defenders can't break through. You can't use traditional anti siege methods, since your city is build below ground, so your attackers will have the height and hence range advantage on your defenders.

So here are a bunch of suggestions you could do

Booby Traps

Just throw down a tone of traps and open the doors. Your tunnels and access points will need to be designed to stop things from rolling all the way down, so your going to need a series of down and up ramps to stop this from happening. Your vikings will file in and hopefully enough of them die to the traps for your soldiers to fight

Poison

Depending on your access to water and food, you could poison their supplies and the resources around you. Vikings shouldn't carry too much food supplies with them and will need to restock constantly to feed themselves so if you make sure there is no food and water around, they will be forced to move away

Sea Mist

On some days your going to get a strong sea mist and this gives you a good change of launching guerrilla operations. Sneak most of your soldiers out, attack the vikings from the direction of other kingdoms to make them think allies are coming and then retreat. You could also use this change to poison their supplies or kill off key leaders.

Narrow Entrances

Your entrance ways are going to be fairly narrow so its evens down the fighting from 900vs400 to just the row of men who are facing each other in the cramped tunnels. This means that if your soldiers are much better trained than the vikings, you could try draw them into the city tunnels and fight them there and win. Sort of like 300 style.

Smoke and Mirrors

As you mentioned before, you could try and use smoke to even up the numbers you are facing. The vikings won't be able to see as much and will likely be surprised by the smoke, breath a bunch in and start coughing. Your well prepared soldiers will have damp clothes over their face so they can breathe and this gives them a quick advantage and build up momentum. If you could build up enough pressure as well and open the doors quickly, you could have an explosion of smoke which engulfs the vikings as you rush out. You could also use this idea with a poison gas when you know the vikings are outside to force them to back out.

No Light

One of the other things you need to consider is the advantage of the dark. If your attacking at night you don't want any fires or shiny things with your troops. They make it easier for vikings to spot you and can give your position away. All your soldiers should be wearing black clothes and their weapons should be covered in soot so they don't reflect any light. If you have every been outside away from the city, its basically pitch black at night. The moon is barely enough to see anything. So you could do things like adjust all your soldiers to the dark, open the gates with a bright light and then extinguish the light. Your vikings having seen the bright light haven't adjusted to the dark yet, while your soldiers waiting with their eyes closed are and have a quick advantage over the vikings who are pretty much blind.

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    $\begingroup$ Thank you very much for your time & response! The smoke or gas building up behind the doors is a good idea, and it's always good to be reminded of using narrow areas or advantage positions to avoid surrounding. But I defiantly like your no light idea that is great, I would of never thought of covering them in soot to darken them, that is pretty ingenious. $\endgroup$ – WolvesEyes Aug 27 '18 at 3:33
  • $\begingroup$ @WolvesEyes I think I remember reading it somewhere else. I can't remember where though $\endgroup$ – Shadowzee Aug 27 '18 at 4:03
  • $\begingroup$ "Sort of like 300 style." You're forgetting who won that battle. $\endgroup$ – RonJohn Aug 27 '18 at 6:04
  • $\begingroup$ No windows, no fresh air (how are they getting the fresh air into the city anyway). Using fire and smoke (are there chimneys?) will be very dangerous for both sides. $\endgroup$ – Sulthan Aug 27 '18 at 10:46
  • $\begingroup$ @RonJohn "You're forgetting who won that battle." Well, in that battle, approximately 3000 people (300 of those spartans) held up more than 100000 people for three days IRL, while causing 20000 casualties (last according to Herodotus, the other two numbers are modern estimates). AFAIK a rule of thumb in warfare is to need at least 2-3 times more soldiers for the attacker to be successful against a somewhat fortified defender, but here the fortifications are actually better than at Thermopylae, so a needed 10x factor is more realistic. $\endgroup$ – P.Péter Aug 27 '18 at 12:40
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Well, I think I get the gist of the military situation. The problem with two entrances, obviously, is that they'll get camped - certainly if they're only as wide as a road. On the other hand, I'll assume the invading parties really do want those riches and not simply bury this underground city & seal it off. They want to breach the defences. I'll assume the following:

  • No cavalry on either side (you don't bring horses to invade an underground state I presume)
  • Logistical issues are irrelevant; neither party intends to be patient enough to starve the other (ignoring the question of what the invading parties have planned in terms of logistics)
  • The underground state does not possess the numbers to face both of them head-on in a pitched battle on even grounds

International relations

Wars should always be placed in an international context. You said the two neighbouring kingdoms are to remain neutral, but that doesn't mean you can't use deception and lies to make the enemy think there's an army build-up on the borders (fake tents, banners etc.). In fact, your diplomats could convince the neighbours to simply station some real troops there without actually getting into a fight. No matter how neutral they are, no one likes the sight of a battle on their doorsteps. The point here is that the invaders have to react by at least diverting some of their troops to meet the threat, allowing you to take on a divided enemy one at a time.

Have the two kingdoms quarrel

I can't imagine two nations as dissimilar as +/- the Egyptians & the Vikings to work together in concert without some miscommunication or animosity. Have some of your agents sabotage camps / ships and pin the blame on the other party. In any case, distrust means you face two disjointed armies instead of an united one.

Let them in?

Turn this ridiculous disadvantage of having a grand total of two entrances into an advantage and allow them to breach the outer defences of one entrance to slowly lure them in. I'll assume your army knows their city better than the enemy does. While the enemies are lured deeper into the underground city, try to break out from the other entrance with your main force & attack them in the rear from the outside. Combine this with some mutual distrust amongst the two invaders and perhaps you could pull off some kind of victory.

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    $\begingroup$ Thank you my good sir! your advice has certainly helped, I really tried to sum up what I did, I'm glad you got the point through all of that. And I really like your idea on letting them in, I was completely oblivious to that idea, doubt I would of thought of it. But it could defiantly work now that you mention it! And pitting them against one another is defiantly another possible option. $\endgroup$ – WolvesEyes Aug 26 '18 at 23:35
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Empty Fort Strategy

As per the good ol' Art of War. Note that this kind of mind games are highly dependent on the respective leaders' reputations and personalities. In Romance of the Three Kingdoms, it worked plausibly because the defending general knew the attacking general was the cautious type, and the defender had built up a fearsome reputation through pulling off multiple victories against overwhelming odds.

Conventional victory

Now for the other extreme, the foolhardy invader easily blinded by riches and glory. This is where your network of small 1 or 2-man (presumably concealed) tunnels come in handy.

What your underground state needs to do is to bait the attacking armies into your main access tunnels. There are a lot of ways to do this. The more boring ways involve display of obvious military weakness. You can use scantily clad pretty girls scampering right in front of them; presumably your Vikings are as interested in rape as they are in pillaging. A spy planted as the enemy commander's concubine whispering in his ear can do that job as well. Honeypots are possibly the oldest trick in the book after all.

Once you have drawn them in, the invader should be in a prepared kill zone. Prevent his exit by planting an armed force at his rear that came out via the small tunnels. Now unleash your deadliest artifice upon the invader; burning oil and Indiana Jones-style giant killer rocks work just fine.

A more extreme version of this involves preparing an entire fake city for the invaders to go into. The fake city is full of booby traps of course. You can unleash a whole array of horror movie style gruesome deaths upon the attackers once they have entered this trap.

What comes after

Now comes the trickier part of bringing about a peace that would last at least in the short term. To achieve this you can use a trifecta of fear, distrust and alliances. A fearsome reputation is the most useful in the short term to directly dissuade further attacks. Impaling your enemies on stakes and leaving them at the coast line for all to see, a la Vlad Tepes is one method, as is eating the livers of your foes with some fava beans and a nice Chianti. Since I've mentioned Romance of the Three Kingdoms earlier, you can also invite their ambassadors to a human meat banquet(tucked in the 'Rule of terror' section)

Distrust has already been detailed by others. Finally, try to establish yourself as a trading partner, if not an ally with your neighbours. A big trading account and potential allies makes it less likely they'll want to invade you again.

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In my answer to the question, How would the design of a habitable underground fortification differ to that of a castle?, I described in some detail the construction of an underground fortification.

An intelligently designed underground fortification won't be as simple as a cave with gated entrances. The entrances will be designed to be actively defended, so if the raiders attempt to force entry or even blockade them, they will be in range of the defenders' defences. An assault on the gates would quickly turn into a massacre for the raiders, and if they have any sense, after the first attempt on the gates, they won't try again.

Castles are force multipliers. They allow a small number of defenders to engage a larger number of enemies. For surface fortifications, the usual multiplier was ten - for each defender, the attacker needs ten men to achieve nominal parity. For an underground fortification, the multiplier may well be higher - perhaps twenty to fifty or even more, depending upon the nature of the fortification and the geology in which it is situated, twenty if it is in soil or gravel, to fifty or more if it is excavated into hard rock. Therefore, the defence force of 400-500 men could be expected to hold off an enemy numbering between 8000 to 25000. The 800-900 'vikings', having run once into the meat-grinder that a well-constructed set of defences could be, wouldn't stand a chance. The best they could hope for is to escape with their lives. Picture this:

The Vikings locate the entrance to the underground kingdom, and prepare a battering ram from a convenient large tree in order to take on the wooden gates set into the cliffside. They advance under their shields as the defenders rain arrows down upon them. The fact that the arrow-loops on the cliff face are entirely enclosed makes counter-attack difficult - an arrow must be aimed to pass through the loop, and siege weaponry munitions are too large to pass through the loops, the only hope is containers of burning oil, but if those were to be used, the oil would burn the siegers more surely than the defenders.

On reaching the gate, the ram is swung, and despite losses, the gates are breached. A few defenders flee down the tunnel and vanish around a corner- those that aren't killed. The raiders follow, only to find that just around the corner is another gate, but this one isn't wooden, it is made from tens or even hundreds of tons of stone, rolled sideways into place. The ram cannot be employed - the passage is too narrow to turn its great length, and no application of strength can roll the gate aside - the defenders have placed a big wooden wedge behind it, and the raiders might as well be trying to push a stone up a mountain.

Then, when the raiders have swarmed into the entry tunnel, they discover why it is so long and lightly gated at the surface - the whole thing is a death trap. The defenders open hatches in the roof and rain down burning oil, rocks and any other deadly substances they have. A little forethought by the defenders would provide grated vents near the main stone gate and any fire within the tunnel would be fed air from there, the downwards slope of the tunnel acting like a chimney to emit the smoke from the flames from the gate, and the draft would fan the flames.

If the defenders are especially nasty and have had plenty of time to work on their defences, they could close a second, hidden, stone gate just behind the remains of the wooden gates that would trap the raiders, making their escape impossible while they burned to death. Defences like this could easily kill hundreds of men, and the raiders only have hundreds Then the defenders could open the outer stone gate again and dare the attackers to advance inwards again over the charred corpses of their comrades.

So, the raiders have been presented with a fortification that they cannot defeat. They could try to besiege it, erecting their own fortifications around the gate to prevent the defenders from resupplying, however, there is the matter of the second gate.

The second gate would be similarly equipped with defences as the first, but in addition, it would be hidden. There would be observation/defence posts overlooking it so that the defenders could see if it had been discovered or if any enemies just happen to be nearby, but it would be placed in an area too narrow to make a good camp, but the gate would look like nothing more than a featureless rock face.

The second gate would allow the defenders both to resupply and to exit their fortification unseen, at night, when they would engage in guerrilla warfare against the raiders, inflicting atrocities upon them at night and whittling down their strength until they have no choice but to flee.

By engaging in guerrilla warfare, a small force can take on and defeat a larger one piece by piece, or force them to retreat to their own fortifications, making their own resupply far more difficult, and should the raiders do that, they are doomed, as the defenders have better supply.

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I see this a mostly the same a a castle siege, with a few pros and cons thrown in.

Use your weakness as a strength

Since you have only 2 entrances, you only have 2 places to defend, instead of a whole encircling wall. This means you can concentrate your forces in 2 spots, instead of being spread out.

I would think a good defense would start with arrowslits in the hillside surrounding your gates. These might have to be recessed a little, so invaders can't easily get above and directly attack you from them. You'll need them to be just far back enough to require attackers to get into the arrowslit to attack you, which opens them up to being shot down. However, you'll need them forward enough to still allow your archers to see and shoot the attacking army.

Next, you gate should also be recessed a bit, so you can have more arrowslits pointing at your gate. Once the opposing army gets to your gate, they should be surrounded by your archers. A series of openings directly above your gate can also allow you to pour hot tar or boiling water on your attackers. What I'm saying is that your gate should be a choke point for the attackers.

This is working off the idea of the Battle of Thermopylae, where a small band of defenders blocked a pass, preventing a considerably larger force (7000 against up to 150,000) from gaining access. Your forces are much more evenly matched, so you shouldn't have much of a problem here.

Natural armor and tunneling

Other pros is that you cannot be breached in the normal way, with ladders, catapult, or other airborne means. You can, however, still be breached by digging or tunneling. Also, your roof can be compromised and caused to collapse on top of you.

The cons can be mitigated, though. Since you presumably have more experience digging, you should be able to collapse any tunnels your attackers dig, by either digging above or below them. Doing this correctly should trap and kill the enemy diggers.

Seeing as this is a large city, your roof should be rock, instead of just compacted dirt, so collapsing it should be pretty darn hard. The attackers would have to work at it pretty long to make that happen, which should allow you to clear your gates, exit your city, and attack the army above you.

You city should be well below the waterline of the surrounding land, so you should have plenty of water, and castles should be stocked with provisions for months, if not years.

Don't breathe too deep

The only real problem you'll have is air. However you refresh your air can be blocked, either physically or with fire. You can literally be smoked out by building a large fire with green/wet organics that produce a lot of smoke. Having a defense around these openings can help, as well as a filter of some sort.

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100 acres is such a small amount of land (695 yards x 695 yards, or less than a sixth of a square mile EDIT: the Vatican -- smallest country in the world -- is larger, at 109 acres) that it can be easily occupied, and the two entrances -- by definition close to each other -- sealed from the outside. Even worse, when you're in a cave, you've surrendered the high ground.

Now, to the answer... there must be a reason your kingdom is down there instead of on the surface like every other kingdom, and hasn't already been conquered: you're mining a mineral that's valuable to your neutral neighbors.

Because it's valuable to your much larger neighbors, there's a tacit agreement between them and you that they'll protect you from each other and outside enemies in exchange for selling them this valuable mineral at very favorable terms. IOW, you're buying protection.

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Given that the defenders are outnumbered and in a strategically poor position, they can only win by cleverness. The problem with a clever gambit, though, is that against all but the stupidest enemies it will only work once, so you have to be sure to catch as many of the attackers off-guard as possible (and have as many back-up plans as possible).

The best way to even the field is to draw the attackers into a trap (see Dwarf Fortress for some ingenious underground trap design), but a trap that kills a few dozen enemy soldiers will not even the field enough to secure a victory before the enemy wises up to the traps and they lose a lot of their tactical advantage.

For that reason, I'd suggest pretending to lose the battle. The narrow roads ensure that neither army's entire force can be brought to bear at once, which means staging a fake rout is easy than it would be otherwise. After the fighting begins, the defenders should fall back past carefully designed traps, and should fall back as far as possible without giving the enemy freedom of access to the city (as that presumably has a more open layout and you can no longer control their movement). Once enough of the enemy are inside, you can use secret routes to encircle them and close the front gates. Ideally you have several gates in a row that you can drop at the same time, sectioning the enemy into small, tidy forces that can be easily overwhelmed, one-by-one (optionally with traps in the rooms to soften them up first).

The fighting needs to be staged well so they don't get suspicious, the fighting can't go too easily for them, and the pace needs to be kept up - you do not want the invaders to have time to inspect their surroundings and find the hidden passages and traps you'll need to seal them off.

If your kingdom has used this strategy before, though, the invaders may be aware of it and act accordingly; there's not much they can do about it other than refusing to enter the city until everyone inside is dead, though (from poison or starvation, perhaps). Even this can be worked around, though, with a false traitor - someone from inside the city who promises to divulge all the secrets of the city in exchange for freedom (and money, lots of money) - who instead just leads them into a different trap.

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The main problem I see is the actual defense. During a siege of a castle, attackers will always try to cut the access to food and water first. That's why every castle/fortified city needs it's own access to water and enough food to survive for several month at least.

When attacking an underground city, the food problem is the same. However, there is also the problem of air. An underground city needs ventilation holes and it needs a lot of them. It also probably needs chimneys.

The first step when attacking such a city would be to find those holes and close them (it will be probably easy to find them during winter because of the heat and vapor). Then, the defenders will start dying quickly.

The obvious result is that the defenders cannot just close the gates, they have to open the gates and let the invaders in and fight inside.

Unfortunately, it the attackers decide to just block the gates and let people in the city die, there is not much they can do. The strategic disadvantages are just too much.

If you want them to win, you need them to start digging and open more entrances.

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