For the most part Western European castles were heated by roaring fires in fireplaces rather than underfloor heating or stoves seen more often in Central and Eastern Europe. read more
How difficult it is to heat up a castle with a fireplace would require some experimentation but the fact that people tended to wear a bit more clothing suggests they weren’t used to 21 degree room temperatures. Of course there were several months a year when castles didn’t need to be heated at all. read more