Any magic from the castle itself were nothing out of the ordinary, a protection spell had been cast at some point. Though whatever was setting off this feeling so to speak wasn't able to be detected or determined by spells. One might describe it as an invisible fog, as you near it it is thin and gives a person a heavy feeling. The closer you get the more weight is felt and even the slightest bit of dread at first, it's not in any way a welcoming feeling to a stranger.