[Norfleet]

The castle has no effect on the effectiveness of Murdering Winter, Flames from the Sky, Fires from Afar, or any such spells: Presumably, any protective benefit that the castle would provide is offset by the fact that everyone would end up gathered in the castle as well.

However, I don't think Murdering Winter is going to work terribly well against a bunch of cold-immune undead. Why not try a helping of Flaming Plasma Death instead, or maybe a drive-by priesting?