Fascinated by everyone's but especially American's desire to give medieval keeps, especially in colder regions, central heating (and I think Winterfell is to blame for this trope, where, to it's defence, the hot springs were not a matter of comfort but survival wrt the deadly fantasy Winter that's not real irl), because I'm always like. okay I know they told you in middle grade that castles were all cold and drafty but like ... no also what
There's generally going to be rooms dedicated to and build for warmth, the living quarters, both for nobles and their servants. This will be the central living tower, or parts of it called a Kemenate (literally 'room with a stove'), the great hall and work spaces around the kitchen. You can put the Kemenate on top of the hall to catch the big fires' and daily living's heat through the wooden floor, but you often can't put wooden stuff on top of the kitchens (that's a fire risk). If you have the money and space, you build a whole separate comfy place for living because you don't have to stay in the most defensible part of the castle all the time. These separate living buildings are also called Kemenate and are often build from wood, cob, brick etc.
People used to wear much more clothes indoors, including while sleeping, and those clothes were much thicker and sturdier than what we largely wear today. Every time you think of how cold those stone walls are, think about everyone wearing a linen shift + two-ish layers of wool on all body parts except hands and head + stockings and shoes + some kind of head-covering. In Ye Old Middle Ages, women are probably wearing a wimple, which is kind of like a modern Hijab in terms of coverage. People wear shifts, socks, and a head-covering to bed.
I think people used to radiators also really underestimate how much a large open fire/tiled stove heats up a room. Also, middle and northern Europe (as well as parts of Northern China) had and to this day have beds and benches build into tiled and cob stoves. Those fuck.
Beds are enclosed so you stay warm in them, either by curtains, in wall niches or with wood. There's also a type of bed that's inside a chest (like a coffin) so you can stuff your stuff inside during the day and put down the lid to use it as a bench. That's also another reason for people to always sleep in groups. Depending on the era, one of the jobs of a lady's maid or a retainer might literally be warming their master's bed. In early times and among servants, people also sleep in large groups in rooms together in general even outside a farming context, often with animals like pet dogs, too, which further warms everything up.
Walls are not bare, cold stone, but covered with a layer of plaster or cob, tiles or wooden panels, sometimes layered, and believe me, this makes such a difference. Source: I lived in a Ye Olde German Farmhouse with 70 cm thick stone walls and flag stone floor and all that converted to modern flats for a while.
On top of that you hang tapestries on the wall, which are not like modern printed cloth but basically wall rugs, sometimes several inches thick, and rugs or rushes (like a light cover of hay) on the floor on top of stone, tile, wooden panelling or a cob floor cover that goes over the heave flag stone. Pillows and blankets on all sitting surfaces, often on top of panelling (in the case of benches build into the stone). The roof of a room is also tiled, panelled or plastered. Upper stories will generally have wooden floors. Stories in a tower heat each other upwards, so the nicer rooms are further up.
The inner stone walls of a castle, even if stone and very thick, will heat up a few degrees in comparison to the outside walls if the castle is continually heated/lived in, and also trap heat inside, and this will make a difference. Inner walls might also be thinner and made of wood, cob or brick. You're defending against the outside, after all.
You put stuff in the windows. Holy shit. Screens of wood, horn, cloth or leather/hide, often treated for extra insulation. Why are these fantasy castles all so drafty.
Like, idk, I know Americans especially can't pop down to their nearby castle museum to have a look around, but even with people who can and do: The castles you'll see, even the ones who aren't 'ruined' are ruins. They're stripped down. I remember touring Norman towers in England, and those places do look dire and are cold because even if they're still standing, they're ruins. It makes such a difference to get to look at a castle that is still lived in, has been inhabited until recently, or has been historically restored where these amenities are preserved. The exact amenities will depend on the era, of course, but they'll be there. The publicly accessible parts of Burg Eltz are a great example to google, especially since I promise you, you have seen this specific castle before. They have pictures on their English language website here, and the German National Geographic has a few further inside pictures here. Seeing a place like that that isn't a ruin with bare, stripped walls, nothing in the windows, no decorations and furniture etc. makes you realise that yeah actually. My characters are probably just gonna go grab a pillow if their ass is cold on the window's stone bench. Blankets are a pretty old technology, humans (elves, dwarves, whatever) can figure that one out.
Oh these links are a FANTASTIC reference!
Remember the painting of Ivan the Terrible cradling his dying son?
Yes, yes, unequalled representation of unspeakable grief and guilt and horror, that's not important right now. Look at how heavily carpeted everything is -- multiple layers of carpets! -- and how heavily dressed they are.
Also in that painting, the object in the background looks like a ceramic/tile stove or heater. They were found all over Europe and are still used in some places (having experienced one in Hungary in -16c weather, they are amazing). They're like a descendant of hypocausts, where hot air was directed to warm specific areas of building.
The fuel was burned slowly and brick and tile structure acted like a giant radiator, staying warm for extended periods.
