A common sight in fantasy is the mountain fortress, a Dwarven castle embedded in a giant pile of rock. In Lord of the Rings, in the Lonely Mountain, lies the city of Erebor, the city in the Mountain, but how realistic is it? Assuming that I have an army of 5000 workers willing to build this castle, how long will it take? What problems would arise and what would the advantages and disadvantages be of building it?

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closed as too broad by Aify, fi12, Hohmannfan, WhataTiberius, Xandar The Zenon May 16 '16 at 21:09

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    $\begingroup$ "a dwarven castle embedded in a giant pile of rock" - this isn't clear enough for us to tell you how feasible a mountain fortress is. There are lots of different types of mountain fortresses in fantasy, so we're going to need more detail than that. $\endgroup$ – Rob Watts May 16 '16 at 16:08
  • $\begingroup$ I agree with Robb $\endgroup$ – Xandar The Zenon May 16 '16 at 21:09
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    $\begingroup$ I recommend Slaves to Armok II $\endgroup$ – dot_Sp0T May 17 '16 at 11:35
  • $\begingroup$ We can and we have. I've spent two days in Petra. I've never seen Lord of the Rings, but Petra has, sure a lot of openings; but also goes pretty deep and was a functioning city. It is built into sandstone. When I google Erebor, that looks like much harder stone. Petra was a city, not a fortress. $\endgroup$ – Mikey May 17 '16 at 17:19

Probably not.

Major problems:

  • Air circulation. Humans breath and fire uses oxygen. CO2 sinks, so if we let the air remain stagnant, then CO2 will build up to lethal levels in the lower floors. Miners carry canaries to detect these build ups. Modern underground structures have regular ventilation shafts to the surface and automated fans to keep air moving.

  • Mass. To create a room with a large vaulted celing underground, you have to move all that stone. Take the throne room in Erebor, call it 200 ft high, 800 ft long and 200 ft wide, in very rough numbers ($= 32 000 000~\text{ft}^{3}$). Sandstone weights about $150~\frac{\text{lb}}{\text{ft}^{3}}$. Say a dwarf with a cart can move 300 lbs at a time and the dumping ground for stone is close enough he can make 10 trips a day. That is 1,600,000 man days or 320 days for your team of 5000 just to move the stone, forget about cutting it out, and forget about the rest of the fortress.

  • Structure. Making 100 ft high vaulted ceilings is very difficult and usually requires building a wooden support structure to hold the vaulted ceiling in placed while it is being assembled. This would be ludicrously expensive, and, more importantly, would be very likely to collapse in the event of an earthquake.

  • Food. There is no sunlight and there needs to be some net energy inflow to feed the community (if you eat fungus, the fungus has to eat something). This will probably require the city to import food, which is expensive, so there has to be a good reason to go underground.

Minor Problems:

  • Pockets, when tunneling you can hit pockets of water, poisonous gas or weak rock, all of which can be a problem.
  • $\begingroup$ Good navvies and locomotive firemen moved 15 tons per day. Dwarves are supposedly stout, so should not be moving only one tenth of what humans can. $\endgroup$ – Pete Kirkham May 18 '16 at 8:41
  • $\begingroup$ @PeteKirkham do you have a citation, or more specifics? How far did they move it, what did they move? Did they use steam engines or other tools? In the case of locomotive firemen they are shoveling coal a distance of maybe 10 feet we need to move the stone at least 800ft just to get it out of the room figure at least another 1000ft to the entrance to error and another 1000ft to the dumping ground. Since we are moving it 280 times farther we will move less than a tenth $\endgroup$ – sdrawkcabdear May 18 '16 at 17:08
  • $\begingroup$ Navies - historylearningsite.co.uk/britain-1700-to-1900/… 15 tons is the commonly stated figure in many railway/canal history books for navvies. Generally it's the lifting that's done by humans as horses were used once the wagons were full; for canals that would be digging, then pushing a wheelbarrow up a plank from the bottom of the canal to over the top of the wagon. Filling a wagon takes a day, whereas using a horse to pull a wagon 2000ft takes ten minutes, so horizontal distances aren't much of a factor. $\endgroup$ – Pete Kirkham May 18 '16 at 20:59
  • $\begingroup$ Its true my question assumes the dwarves are pushing the carts by hand, so the limiting factor would be horizontal moves not loading. Horses would speed this up though they would have to haul a ways uphill since Erebore is underground. Now I need to think of a metric for how much stone a dwarf can chip off a wall in a day. $\endgroup$ – sdrawkcabdear May 18 '16 at 23:22

Modern engineering and machinery enables us to build some pretty impressive things. Emphasis on modern.

However, the city of Erebor, as shows in the LOTR movies is ... gargantuan. Vast ceiling supported by ridiculously tall columns, massive staircases over lava lakes, halls that would put large airport terminals to shame, etc. Even with our technology that would be an undertaking that would take decades (I almost want to say centuries).

When digging within a mountain (not necessarily under sea level, which brings problems of its own) you have to consider that any sort of earthquake, etc. would potentially cause massive collapses if you were to try and build these massive halls. You would need to reinforce them to a ridiculous degree.

I sincerely doubt that 5000 workers with medieval tech could pull off something even a third of the size within their life times.

Some sort of mountain fortress? Sure. Maybe even a pretty cool one.

But Erebor? No.

Also keep in mind that in most LOTR lore these dwarven cities are depicted as having been built over generations and generations. And dwarfs live for hundreds of years. If you were to combine a long timeline with some sort of dwarven magic you might be able to explain it.


We can most certainly build such a structure. We build all kind of structures underground and within mountains.

check out these idiots!

And take the virtual tour of their temple.

We build tunnels for trains and cars through mountains, and Cern itself is a 27 Km long tunnel hundreds of meters below ground.

To be fair these are examples of very advanced engineering, not medieval level tech, though it's worth noting that the Damanurian temple was built by amateurs

EDIT: Ok so Erebor is really REALLY big, It's a lot of material to move! So either you need more people or a lot of time. Maybe the fortress it's the result of centuries of work, but it's probably possible at least from a technical point of view


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