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Assuming a world otherwise like ours and at the same technological level of medieval Europe, what change in the world might have caused people to build defensive structures underground rather than above ground?
I've already thought about riding large flying creatures, and dragons are far too obvious. What else might cause this?
The short answer is nothing. The offensive and defensive aspects of war tend to evolve in a leap frog fashion in much the same way that any two directly opposing forces do. Without a threat to necessitate the development of underground defenses there wasn't much drive to create them.
The best defense at the time was high walls, gave archers better range and line of sight, and it made it harder for attackers to get through. In the end defense are there to keep you protected and applying force as long as possible before the enemy is upon you. If I think about warfare at the time, underground defenses actually seem like a worse idea than what medieval Europe actually had, for example fire was a commonly used weapon...underground + fire = bad.
A fictional threat would be the only thing I can think of that would necessitate the creation of underground defenses in medieval Europe...more to the point, without that threat they would actually be less effective in medieval combat when compared to castles, towers and walls.
Flying creatures of some kind are the obvious choice; dragons, wyverns, maybe some giant Rocs.
I suppose you could go with some sort of really large land animal, I am thinking of the elephant/hammer head shark crossover from Avatar.
Giants. Really really big giants.
A magic system where the earth somehow negates powers...
Now, even with these creatures in the world they would have to be common enough that they showed up in a good portion of battles, otherwise, when its just human on human fighting castles would still be the way to go. Castle building/defense in this scenario would, I think, be mainly for hiding as opposed to castles traditionally being bastions from which to exert your local authority.
edit: Thought about this some more, and no matter what else a castle needs to allow you to fight back otherwise it is just a tomb, doing this on a flatland...I don't see that working out well for the defenders. If the castle is underground...returning fire would be pretty tough, unless its built into the side of a mountain or under a hill...like this.
So in this image, you could have an underground castle with tunnels up to the surface, if your enemies are dragons, their are trapdoors with ballista, or maybe a good old fashioned death trap for a giant (big spikes at the bottom of a hole). I still think a flying enemy makes the most sense and when I think of underground fortresses I do feel like its more hiding than just purely defensive like a castle.
Medieval Europe had a greater dependency on existing features of the land than we do now. Cities often grew up around rivers and fortresses were built on existing hills.
If the ground was riddled with natural caves and tunnels then large cities were established underground, such as those at Derinkuyu and Özkonak. These date from 500-1000AD, they housed tens of thousands of people (very large for the time), and they were well fortified.
If you're looking for a change that forces every fortification underground, why not make natural underground features much more common? If living underground is the social norm, and naturally defensible features such as hills and rivers have useful underground features then the fortresses will naturally be constructed mostly underground.
A climate far to harsh to stay on the surface for any length of time.
EDIT: These ideas don't stop people going on the surface necessarily. Just not for too long.
What about hot-air balloons or zeppelins? They have been inserted into middle age with success in some stories taking place in the middle ages.
They are a big advantage for attackers (if they manage to navigate over the enemy place), and would allow them to "jump over the walls" (thus justifying your underground structures), but at the same time they do not require a really advanced technology, and is not too unbalanced since they could be knocked down by an archer (specially dangerous if the arrow is in fire*)
Air-transport would also make inefficient walls guarding a territory. such the Great Wall (just fly over them) or castles in the middle of the way. This will mainly depend if provisions can also be transported that way or they need a traditional ground convoy. Also if there is a way for defensive balloons from the castle to intercept the attacker ones (can they block another balloon, or only move in the same direction imposed by the wind?).
* The balloon will explode, additionally putting in danger other attacking balloons.
In Anne McCafferey's Pern books, humans retreated into underground cities to escape Thread, which was an organism like a highly corrosive string that fell from the sky from a rogue eccentric planet for 50 years each 250 years. Since it would devour animals and vegetation, but not stone, people had to live in stone dwellings. Of course, the dragons in the series helped make sure that thread didn't fall to ground, but they weren't infallible, hence underground cities.
If the planet habitually flew through a light meteor shower. These would need to be bad enough to cause light destruction and kill the odd person, thereby necessitating the building of underground defensive structure much like air raid shelters.
However they would need to be not severe enough to do major structural damage to the society that might impede technological advancement of the civilization.
I'd venture to suggest that the tunnel systems used by the Japanese during the battle of Iwo Jima would be a viable tactic in a medieval setting against a force vastly superior in numbers & equipped with such siege engines (including cannon, mortar, bombards etc) as would render a castle little more than an expensive trap for the occupants.
Where you have a small number of defenders equipped with ranged weapons (such as bows or crossbows) or who wish to make harrying attacks on the enemy in a highly mobile fashion, popping up in unexpected places & vanishing having ambushed the enemy, an extensive tunnel network would be very useful indeed.
All kinds of possibilities, really.
Underground defensive structures can be an outgrowth of escape tunnels (for example, while facing a large invasion army while having a very small army on your side). Even with small or no changes to how physics works, people would still use and develop technology along existing lines of development - so, for instance, if people used to make overground structures, but some warlord employed technology and strategies taking advantage of their weaknesses so severely that they became useless, forcing people underground, it could explain why you'd see a lot of them around. If others imitate the same methods and technology (very likely if they don't have the time or means to come up with better ideas or the application of other methods doesn't catch on enough, considering underground structures are the target now) it's likely that you'd see widespread use of underground defenses, possibly even adaptations and structures not employed in real life due to lack of need (no need for us to have ways to mitigate liquids poured into shafts or vibration siege mechanisms in the 14th century) - we'd come up with sewage systems and flushing toilets sooner I suppose.
They could also be a side-effect of landscape features or lifestyle. If there's a lot of sharp elevation changes and/or a strong need or adaptation to mineral and metal use, chances are lots of people would want to stay closer to their work or prefer tunneling into a mountainside instead of building a castle on a slope at a large angle. Obviously, a large mining colony underground would require safeguards to not die from fumes and dust, but people are resourceful enough in fantasy settings :P
Not really suitable for a mediaeval fantasy genre, but a lot of Moonbase scenarios involve burying pressurized shelters under regolith. Large quantities of dirt and rock can make an excellent radiation shield. This could be seen as a variation on "blistering heat on the surface." Just a different definition of HOT!
The weight of soil and rock could be a hazard in itself. On the other hand, a properly designed shelter could be less sensitive to blowouts when buried. One assumes the shelter would be properly designed…
because of:
My suggestion, entirely fantasy based:
Everyday at night (twice a year on eclipses) the moon of -insert name here- rises in the sky and basks the land in a red light. All land animals become agressive after a few minutes of full exposure.
The red light comes from a distant sun directly opposed to the main sun. Its light reflects on the moon and turns the night a dark blood red color. On the other planet side, the daylight from the main sun cancels the influence of the red sun.
The red sun isnt visible during the day, but sometimes the moon has a pale salmon color.
It doesnt affect water animals, as they´re not directly exposed to the same amounts of the red light.
All land animals that hunt at night and get exposed become agressive, somewhat rabid like for a few hours.
The red light does affect humans, and more agressively as they dont have hides. The usage of thick cloth is required. Also, many live underground, as it is common knowledge that it is the best place to shelter from the evil influence od the red eye.
So, many children stories warn about the dangers and the monsters after dark.
Also, on eclipses, the moon blocks the main sunlight and reflects a lot of the red sun light, the influence peaks at this moments and these are days to be feared.
Dont crucify me on lack of astronomic details or such, I´m just trying to present a new view for the question (They only need to live underground during the night).
Hail
Even in modern times hail can cause substantial damage to buildings and automobiles. If something happened to cause frequent storms with baseball or larger size hail, building underground shelters could easily become the de facto solution. It would be easier than lifting and supporting huge sheets of stone.
Advanced/Giant Catapults
A mobile war machine that can rain giant chunks of stone down would also necessitate heavy overhead protection.
Pacifism
Castles may be defensible, but they are also obvious. If the city in question was more interested in hiding than fighting back, going underground is the more obvious choice.
Underground defenses or building did, in fact, exist in Medieval times.
This was because underground offensives existed. If you couldn't get over a wall, you'd tunnel under it. You started your tunnel out of sight of the people behind the wall, because then they would make a counter tunnel to prevent it.
Tunnels were dug in order to take down walls, you'd tunnel underneath a wall to weaken it, allowing your army through. The counter tunnels were dug to meet them before they could, and kill all the men in the tunnels.
If it happened often enough, some castles had lots of tunnels under them, already built and shored up so that they would be ready to meet the offensive tunnels, and if the enemy got in the defensive tunnels, they would be met by soldiers.
There are other reasons for underground building. Storage. And protection from the weather. Abbeys would often build them, just to stay out of the harsh winter.
Everyone else on here had had some excellent ideas for forcing things under (natural features, outdoor problems) but just wanted to point out that underground defenses did exist back then and the reason.
[[https://en.wikipedia.org/wiki/Derinkuyu_underground_city][1] and https://en.wikipedia.org/wiki/Kaymakli_Underground_City]2 are you best examples, built when the ground made it easy and periodic raides were the problem not large scale military incursion. The big benefit it is is really hard to break in. they used a giant stone wheel they could just roll over the narrow entrance from the inside, and there was basically no way through it short of spending months with chisels. great for places with little to no military. In the desert underground is also a lot cooler during the day and warmer at night. of course they had buildings above ground as well.
if you have big flying dragons, this would be perfect defense for villages and towns, there is no way for the dragon to get in, just too big, and you can hide the livestock inside as long as you have stored food. larger cities would probably build strong buildings, but you might be able to justify it if the threat is older than the cities, and dig down was already how they dealt with the threat. they might never have considered other options.
Your alternate world would need to have easily available helium, that's all.
People will figure out how to make a balloon quickly (without the need for electrolysis or other hard tricks to obtain a light gas). The invading army just needs to wait a couple of days and once the wind is good, they launch bomber balloons. The bombing goes at about 200-300 meters altitude, so it is hyper-precise even with the most rudimentary bombsights. No optics needed from such a distance. There is no active defense possible with medieval weapons.
The city walls don't make sense, they take decades to build but a breach could be made with just one gunpowder bomb. Just a single one-man balloon with a single 100-200 kg bomb can open the way for a regiment of infantry.
You need underground bunkers for military and large shelter systems for civilians. You can release the infantry anyway when enemy infantry is at melee range or when a last balloon passes. Archers can shoot from within the bunkers.
The defense forces could use cavalry a lot: it doesn't need to go to the shelter at all, because it could outmaneuver balloons with ease.
You need helium not hydrogen, because the balloons need to carry an open fire for those gunpowder bombs.
