IRC the Magic Armor is magic because it is GIGANTIC and also randomly appears places, like all of a sudden there will be an enormous helmet in the Great Hall or whatever, but it has been a while since I've read Otranto so my recollection of the details is a little fuzzy.

It was a couple of years ago for me too, now, but that is pretty much how I remember it too!

I should re-read it because my instant reaction was that it should be a YUletide fandom if it wasn't already, but then I got distracted by Mannerling the randomly evil stately home from MC Beaton's fic instead. (Tough choices have to be made sometimes.)