In the remake, sure, the explanation of why rusted iron is the demons' weakness makes a certain amount of sense, but why did they feel the need to make the weakness rusted iron rather than the typical just iron? I mean, there's plenty of currency in demons (a chaotic, wild force) being vulnerable to iron (a symbol of man's industry and imposing order on reality). It's not even necessarily harder or easier to find one or the other. So... why?
Iron is a little easier to come by, I would think. At least from the audience POV who probably assume more things are made of iron than there actually are.
Also, in the remake, why are there norse runes in with the spells on the wall? Did these demons play the part of frost giants back in ancient times?
Could be the origin of the magic is irrevelant. As long as it's anti-evil, it'll work... yeah, reaching.
They may well have played the part of evil spirits. The basic archetype of possessor/corruptor-via-sex monster is a fairly common one in mythologies... including Nordic folklore.
Finally, from this troper, at least, traditionally evil spirits cannot interfere with the script of a barrier spell. And yet the demons could make the walls of the protected room bleed, which in turn began washing the spells off the walls. So... basically, no standard rules of magic apply to this movie's universe, is that right?
Yeah, that and the scene where Angela moved some furniture in the room bugged me. My only guess is the runes weren't one-hundred percent fullproof; they could keep the demons from entering the room, but weren't powerful enough to stop them from effecting stuff in it. Either that, or being left to rot along with the rest of the house for close to a century weakened the spell just enough; smudging, scraping, etc. from regular wear and tear just wiped out some of the marks.