What kind of charm could possibly be out there to retain the castle’s heat?

The little Ravenclaw girl had the right idea. Entrapment. An entrapment enchantment. There were a multitude of spells they could use to keep the area warm it was just the matter that it was so fleeting. The ball of drab and dark laundry that was Cutty Mordaunt raised his hand. "There are protective charms which keep things locked in so to speak. We could use one of those in conjunction with an atmospheric charm as was mentioned earlier. Booht,..." And here was the problem, as Fuller hadn't used a plural in the question. "There isn't really one single charm for a job like this, is there?"

But so long as they required compiled effort, Cutty was feeling partial to potions at the moment and their related effects. "Yew could try ingesting warmth with potions and things like fever fudge." Dilluted of course if the formula was more than what you were looking for.