In Game of Thrones Winterfell is a large castle that is built on a set of hot springs that provide warmth and even keep the castle comfortable during the years-long winters that characterize the region. The hot springs also warm a greenhouse, which provides food when the fields are fallow.

But how does this work, exactly? And can a greenhouse provide enough to feed the population of a small town or keep?

Hot springs were known in both the classical and medieval worlds. Towns sprung up around them, as people went for their supposed health benefits. (In modern Germany, many health insurance companies will even pay for a visit to a spa).

There is in fact, a town in France that got its entire hot water supply from hot springs – Chaudes-Aigues. The original system was built in the 14^th century, supplying hot water to homes with simple wooden pipes. In that case the sources were upstream of the village, so it was gravity fed. The water comes out at about 82 C (179 F) which is a lot hotter than most homes (which are about 48 C, 120 F).

Historical castles weren’t often built on or near hot springs, mostly because the springs didn’t happen to be in important enough locations. Remember castles were a military installation first. But it’s not hard to imagine that in Westeros, Winterfell happened to hit a spot that was strategically important.

So how would a medieval stonemason work out an in-wall heating system? The Romans made pipes of lead, and the technique for making them out of wood and other materials was certainly available to medieval people. Clay pipes were known as early as Babylon. Contrary to popular myth, the Romans weren’t sickened in by lead poisoning from the pipes, because the local supplies had a lot of calcium in them. If you live in London or Los Angeles you’ve seen the effect on your tea kettle: scaling. Calcium carbonate formed on the insides of pipes, and that actually kept a lot of the lead out of the water. The lead poisoning the Romans got was largely due to lead as a food additive to wine and the use of lead implements in cooking.

As for pumping, medieval engineers were familiar with the artesian well, which was worked out in the 12^th century (the name comes from the region of Artois). If the hot springs of Winterfell are heated geothermally, and the water recharges a local aquifer, it’s not inconceivable that near the source, the water would emerge at relatively high pressures. (The good people over at Generation Anthropocene have an outline of the geology of the hot springs, which may resemble those of California).

It’s unclear from the books whether the people in Winterfell or the local Winter Town drink the hot water; that from Chaudes Aigues has a lot of salt in it – as much as 5.9 grams per liter –and wouldn’t taste very good. The geothermal water in many parts of California isn’t really potable either.

So we can imagine an early architect of Winterfell seeing some local hot springs, and either using the pressure of the spring itself, piping the water downhill (if there is enough elevation) or digging an artesian well.

The walls that are heated only have to be in the parts of the castle where people live. According to a Wiki of Ice and Fire the Great Keep is the part of the structure built over the hot springs. Besides, there’s no good reason to pump water through the outer curtain walls, nor storage areas. So it is perfectly feasible to imagine Winterfell’s builders – perhaps Bran the Builder himself – designing a keep with thick outer walls for both defense and to keep the heat of the hot water in, and a thin inner wall to allow the heat to radiate out. That designer may have even set the system up as loops of pipe (or even simply openings in the stone, similar to the Roman design for many aqueducts), allowing water to flow downward after it has cooled, and then back to the hotter water that comes from the spring – a giant primitive heat exchanger. A medieval technician, if the person remembered Roman-like technique, could certainly build something to supply Winterfell with hot water.

Also, if you want to keep a room warm, you needn’t heat it that much. Assuming a water temperature like that of Chaudes-Aigues, one could imagine walls radiating heat that is enough to keep a room pretty comfortable. Old-fashioned steam radiators were of cast iron, and they could keep a room at 70 degrees.

Doing some simple modeling, and assuming that the heat conduction for granite,is about what it is for Scottish granite (Per Wikipedia), and a wall area of about 10 square meters, 10 centimeters thick, and the temperature difference of 82 C (basically heating up a room from freezing), the total heat loss would be on the order of 3800 Watts, which is plenty to heat a room four meters on a side.

But what about food? Winters are years long, and that means you need foods that keep in a cellar or storehouse without rotting (to say nothing of the problem of vermin).

Winterfell has a greenhouse. As far back as the 13^th century Italian lords built botanical gardens. And the Romans had the technique of planting cucumbers and leaving them outside to get the sunlight while bringing them in at night. Romans architects and engineers also occasionally used selenite, a transparent mineral, as a kind of  transparent windows. Medieval churches are, of course, full of stained glass. So greenhouses were possible with medieval-level technology.

Let’s assume Winterfell has a garrison of some 200 people – originally the Stark family, plus their most trusted retainers and knights. (Ned Stark took a contingent of about 100 people with him to King’s Landing, and that was a sizable part of the castle’s manpower). In actual medieval castles most of the people working there didn’t actually live in it unless there was a siege, but George R. R. Martin has the Winter Town outside the walls, so we can assume that the castle might also have served to store provisions for the long winters. He doesn’t say how big the Winter Town is, but most medieval towns were small, and the North is said to be sparsely populated.

A greenhouse doesn’t need to be quite so large; if the purpose is to provide supplemental fruits and vegetables. But heat isn’t the only thing that plants need; they require sunlight as well. Many herbs, for instance, won’t grow well at all unless there is a solid six hours of sunlight at least. It isn’t clear if in the world of Game of Thrones that the nights get longer in the winter, though there is some indication that they do. If so, the greenhouse won’t be as much help as it might be.

A single person eats about a pound of wheat per day (this is about what it takes to make a loaf of bread), so that’s about 365 pounds a year. So to feed a village of 5,000 people would require some 1.82 million pounds of wheat per year. A bushel of wheat weighs in at about 60 pounds, and that means about 30,400 bushels, or on the order of 37,000 cubic feet of space. That fit in a room about 40 by 20 by 50 feet (extrapolating from numbers here).

That isn’t a lot of space given the size of the castle of Winterfell, which covers an area hundreds of yards on a side. So it’s not hard to picture storing enough food for thousands of people for two years or more. Wheat stores rather well (most plant seeds do) as long as they are in relatively cool (but not too cool) conditions. A storage area that can be kept above freezing should do it.

However, wheat isn’t the only food available. Martin also posited potatoes as part of the Westerosi diet. Potatoes store for months in a root cellar, and even when they start to sprout a bit you can simply cut those parts off before you cook them.

Other vegetables don’t store so well – tomatoes, for example, can be dried and last several months, but not years. This is where the greenhouse comes in. If you wanted to use it as a supplement to the bread, porridge and boiled potatoes during the winter, it might just work. The “glass garden” dimensions are never explicitly stated. But roughly speaking if the glass gardens cover a hectare (10,000 square meters, or 0.4 acres) it’s possible to grow enough vegetables on a rotating basis to at least supplement the diet of the garrison in the castle itself. Even with relatively primitive farming methods one could plant the whole greenhouse with about 3,000 tomato plants, for example, which is enough for a lot of people to eat. On a rotating bass one could plant onions, which last a couple of months with good storage conditions, or carrots, which last anywhere from two to seven months in a root cellar.

Looking at the shelf life of most vegetables, though, it seems pretty likely that once winter sets in, a greenhouse would have to have a pretty rapid rotation of crops, though there’s a lot of leeway depending on the vegetable. It also depends on the sunlight; even if the temperature is right it is hard to get crops to grow right absent enough light. So if in Martin’s world the winter days are shorter the population of Winterfell is going to have some problems even with a greenhouse. Essentially, the diets in Winter Town are going to get mighty boring once winter gets into year two.

That said, home grown food isn’t the only option. According to the histories mentioned in Martin’s work, the southern Westerosi regions did have some food surpluses even in winter; the seasons might be years long but they don’t grip the world in an ice age, either. So it’s certainly possible for Winterfell to import some food. That would be crucial; even with years of shelf life for wheat and other grains, any winter that pushed the five year mark would be a real problem for the people of the North.

While we seem to have solved some problems of heating and feeding Winterfell, it’s useful to ask why medieval engineers never did this. The reason is that in some ways it is a problem in search of a solution. One of the big medieval innovations in home heating was the fireplace and chimney. Putting a fire up against a thick outer wall, surrounding it with stone that could re-radiate the heat, while the smoke and soot went out the chimney, worked very, very well and was far superior to the previous system of putting a hearth in the middle of the room. It’s also a lot simpler to build. Castles were first and foremost military installations, not homes. The fanciful castles of Germany like Neuschwanstein were mostly built in the 19th century.

This is also ties into why greenhouses, though they were known to people of the period in basic forms, were rare. The great botanical gardens of Spain (as in Seville) and Italy were usually outdoors, rather than enclosed. Clear glass in large sheets was devilishly hard to make – the pieces that make up a stained glass window are relatively small. So if you built a greenhouse it would probably not have the huge panes we associate with modern designs. And while selenite can substitute for glass, selenite is a crystal, and very hard to cut into sheets.

Window glass, in fact, remained something of a luxury all the way to the 17^th century. Enclosing an area of any size in glass would have been a hideously expensive project to any lord or king of the middle ages, even allowing for glass that wasn’t clear.

Note: this post was edited to reflect that Artois is a region, not a town.

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If you liked this post, and like Game of Thrones, check out the following Game of Thrones science posts as part of this week’s GoT Science Blog Carnival:

One Reason Scientists and Science Writers Want to Talk About Game of Thrones by Matt Shipman

Winter is coming: climate change and biodiversity beyond the Wall by Jacquelyn Gill

Biology Would Leave the Game of Thrones Dragons Grounded by David Hone

Dire Wolves Were Real by Brian Switek

White Walkers: a Warning Letter from North of the Wall by Michelle LaRue

The Epidemiology of Greyscale by Tara C. Smith

Tales from a Westeros Geologist by Miles Traer

A Storm of Chemistry by Raychelle Burks

Further reading:

A lively Reddit discussion on the use of hot springs to heat Winterfell. I assumed the water wasn’t high pressure steam, though.

https://www.reddit.com/r/AskHistorians/comments/2lrfuw/could_castles_have_been_built_over_geothermal/

Cathedral Forge and Waterwheel: Technology and Invention in the Middle Ages, by Frances & Joseph Gies

Engineering in the Ancient World, by J. G. Landels

22 Foods You can Store in Root Cellars

Neat infographic on how much food it takes to feed a family, and how much land: http://visual.ly/how-big-backyard-do-you-need-live-land