How to live in an underground city?

Question

A group of refugees (around 50) fled enemies in a land full of rock. Fortunately they found exactly what they needed: a small river falling into a deep and tight gorge.

There is a network of caves starting from the gorge, large enough to live, to make fire, to walk several minutes in every direction. Some of the caves are large enough to build walls to separate small habitations. It's not very visible from the outside at first, but my refugees could make some layouts to improve their life conditions and change this fact.

They start to improve their "city". They have lots of water, they find enough food (yep, they do, I don't ask about that; there is a little bit of magic involved), but they know other refugees are coming. Lots of them.

They know they're not a threat to their enemies for now. They are only a few, hiding, and very bad at fighting, medieval technology (like their enemies). But with other refugees coming, they are going to become a major issue to their enemies. They have to secure their place, while being more and more numerous.

What can they do to

• host more and more of them in an underground city?
• secure the place against an incoming army (they have time to prepare)?

Answer 1

The number of refugees that can be supported is depend on space, sanitation, and food as water is already taken care of.

Living Space

Humans are capable of getting by with incredibly small amounts of living space per person. How many people can fit in a square meter will depend on the technology and culture of the refugees. For example, multilevel hammocks allow many more people to sleep in a single room than sleeping on the ground can support.

This graph seems to indicate that humans need about 100 sq ft per person in a medieval setting.

Perhaps this can be compressed down to 50 square feet per person or even smaller but this is likely to be an unsustainable position.

The only solution is to find more caves or dig out new chambers. Perhaps the magic that provides food can also perform some digging?

Defense

In order to defend the caverns, the first batch of refugees will need to prepare for a siege. This means stockpiling food, water, and other supplies.

Water Attacks - The attackers could build a dam down river to cause flooding inside the caves, thus flushing out the refugees. Alternate paths for the river of intra-cave dams will be required to prevent this kind of attack.

Smoke Attacks - The attackers could place fires over any air vents then add noxious plants to the flames. Depending on the plant, this may outright kill the refugees or force them to exit. Many air openings and the ability to block airvents will need to be built in.

Plague - "Someone's poisoned the water hole!". The introduction of poisons or disease into the water supply can cause many or all of the refugees to become ill and unable to defend themselves. Not sure how to defeat this kind of attack with medieval level science/tech. Maybe a chance observation that people who boiled their water first tended to not get sick?

Physical Attacks - Caves have natural choke points in the form of entrances. If these entrances can be heavily defended or completely blocked in the event of an attack, then this gives the refugees a significant advantage over the attackers. Siege weaponry isn't terribly effective against underground fortifications.

Answer 2

How to support population?

Assuming these refugees are untrained peasants fleeing an enemy army, the biggest problems I can see with using a natural cave are ventilation and stability. A badly ventilated cave that is starting to fill to capacity will push some people farther and farther back into the cave tunnels. Caves are dark, moist, and have a tendency to have shard drop offs and extremely narrow passages. This combined with the waste of possibly thousands of people is going to make this camp even better at transmitting disease and causing death than the average medieval town.

Without knowing how many caverns there are, how they are distributed and what their width is I really cannot calculate the number of people it can hold. I can give you some basic ideas to consider. The average worldwide height seems to be 160 - 175cm which is about 2 meters. The average width seems to be 40 to 43 centimeters or half a meter.

If each cavern is 6 meters high and (let's assume) 100 meters in width, we can fit a rough maximum of 200 people in a cavern (without stacking them). These are not ideal/healthy conditions, so let's give each person 30 meters of space. This equates to 3 healthy people per cavern, with the possibility to move this up to 6 people per cavern assuming two people per 30 meters.

Multiply this by the number of caverns and you have the rough sustainable population of the "city" assuming you can provide resources. You would also have to organize "waste teams" to haul human waste to the surface and dump it away from the city (and preferably the river since you don't want to be chugging piss-contaminated water). This job would be undertaken by the majority of people. Anyone with special knowledge (e.g. Magic users, soldiers, weapon smiths, wood workers) would be assigned to teams that utilize that knowledge. This maximizes productivity and will be improved on over time.

Magic can be used to drill ventilation shafts.

How to protect against enemies?

This depends on several factors. What is the expected size of an attacking force? If, say, the cave has a population of 200 peasants and the attackers number 10,000 knights then you are screwed. If there are a thousand peasants, 500 former feudal levies and 500 smiths, you stand a chance in a complex cave network. The best weapon you can make: a spear! Spears are easy to use. Put your carpenters to work crafting wooden poles while your stone workers and smiths create stone and metal spear ends. This way, you can arm your entire population with an efficient, easy to use weapon.

You could also use a bow and arrow (simple bows are easy to make. It's not like you need a longbow or compound bow specifically. Just something that can fire arrows!) and crowd your would be archers into narrow tunnels. Any enemy who tries to get to them will get a face full of arrows and spears. The goal will not be to stop the enemy from entering the cave, but to spend your time (before the battle) exploring and learning every cranny and nook to hide in. This way you can conduct guerrilla warfare in the cave. Make enemy casualties become soon high that they do not see any worth in pursuing your peasant army farther.

Answer 3

I would address both questions with the same solution. To host more and more people underground mine the rock (carve out chunks) where you want another room or where you wish to enlarge a cavern. With the mined rock, you can then build defenses (walls, rock-fall traps, ammo for catapult/slingshot, etc).