Maybe it's kind of obvious, but I didn't realize until today that Morgoth's abode in the North probably had resonance with medieval works (or if I've heard this before, I've forgotten it).
In one of my classes (on Old Frisian), we came across the phrase northhalde trēin a 13th century text. This literally means 'northward tree', but actually is a kind of kenning for gallows. My professor explained this as reflecting traditional lore about the North: it's the one part of the sky that the sun never visits, and is therefore unlucky and the home of evil things. He said that some medieval sources called the North the home of the devil.
I didn't ask him what sources say that the devil lives in the North, but this seems a rather awful lot like Morgoth's dwelling there. Are there any writings about Tolkien that comment on whether this might really be at least partly a medieval derived idea?
(Presumably anit-north sentiment also existed is less Christian sources too. The Norse poemVǫluspácontains a bit about:
[The seeress] saw a hall standingfar from the sunon a shore of corpses;its doors faced north.Vǫl. 37
This hall is the abode, not of the devil of course (since this is an essentially pagan poem), but of the dragon Niðhǫggr, and a kind of hell for oathbreakers, murderers, and seducers (whom Niðhǫggr devours).)
It is hard indeed to believe that one of so great wisdom, and of power—for many wonderful things he did among us—could perish, and so much lore be taken from the world.