In this story I'm giving a go at, humanity as a whole have been under siege from creatures that can be best described as humanoid demons. Things haven't been going well for the human race, as the demons have greater physical strength, size, pain resistance, and seem to all have a sadistic love of violence.
One advantage humanity has over demons is a greater resistance to freezing temperatures. In simple terms, demons will freeze to death faster than humans will. As a consequence of this, humans have started to build towns, castles, and research installations in very cold places to protect them from invasions. However, this has only slowed the conquest of the demons, not prevented it. The humans need a standing army for defense, and demons have no problem changing their equipment to combat the cold.
One of my major concerns is steel, and like metals, becoming brittle with prolonged exposure to cold. Even if I'm wrong about the severity of the brittleness, there is also the issue of trying to wear an ice-cold steel suit in the snow. Hypothermia and frostbite are no joke. To the best of my knowledge, actual arctic warfare was rare in these times (since it was better for both sides to wait until spring) so I'm not sure if there are historical examples I can use.
I'm all for guerrilla warfare with camouflage and mobility being the priority, but I'd love to see how extreme pure defense could get in freezing conditions. Heavy and durable armor is what I'm interested in if that's even possible with kind of climate.
If greater specificity is needed for answers: imagine around 14th century European level of technology and -23°C to +4°C (-10°F to +40°F) temperature (with significant snowfall and wind) to deal with.
What manner of weapons and armor would be suitable for freezing conditions in the middle ages?