The brittle cold that permeates the already laboured walls of a castle can only be described as arctic. It pierces the watchful stone and ekes out of silent corners. It strips your skin to bone, while muscles gripped in its vice tremble against insufficient blood flow. Aching teeth slap together beside an exhausted jaw. 

I think I'd like to explore this one again. I think I could create a more visceral scene of the cold of an ancient castle, and how it feels so intense. I'm going to sit on this for a bit and see if I can find another moment to tackle better descriptors for it.