Matthew.

I had a similar thought when I read about the Mount Tambora eruption of the 1800s a while back, but I wasn't familiar with the volcanic winter of 536; as you suggest, I think some of the surviving historical accounts might have given GRRM some creative inspiration. At a minimum, I think such accounts could have inspired the general "feel" of GRRM's ice apocalypse, but just as there are some potentially interesting prospects related to comets and impact winters with regards to the Long Night, I feel similar potential exists for a volcanic winter. I don't have a specific theory in mind, but a volcanic winter is the sort of thing that could potentially be connected to either the ascent and decline of dragons (eg, activity around the Fourteen Flames), or to the magic of the CotF; for example, if the magic of the Old Gods could be used to instigate the Hammer of the Waters, perhaps it could also be used to induce abrupt and catastrophic volcanic activity.

As you know from prior discussions, I don't really agree with you regarding the 13th LC--not that I think he's definitely still around, just that I believe that the continued existence of the NK is a viable prospect, in the same way that I think the other Kings of Winter could still be out there, assuming that some (or all) of them became Others. That said, to your point about the Nightfort, I also feel that the longstanding "Stannis becomes NK 2.0" theory is plausible, and that that Melisandre comment adds a bit of narrative momentum to that notion. He has lost more and more of himself to fuel Melisandre's shadows, Melisandre is eager to perform a sacrifice, and Stannis's cause seems destined for disaster--regardless of whether he wins or loses at Winterfell, he's not going to have the resources to push south or defend the Wall, which seems to all be setting the stage for Stannis to make a desperate choice at the Nightfort. There's also the HotU vision of a blue eyed king whose sword glows like sunset--rather than glowing like the dawn.

This is the way I interpret that passage as well, which I believe is further supported by the fact that Thistle has clawed out her physical eyes--Varamyr is being 'seen' in a magical and spiritual sense, and he can feel that the thing that has raised Thistle is looking upon him. Later in ADWD, Melisandre 'sees' Bran and Bloodraven in her fires, so the concept of magic users, dreamers, and seers glimpsing one another is not unique to the magic of the old gods, but even so, I believe that the overall context of the Varamry prologue - elaborating on skinchanger powers and taboos, the moment where Varamyr's spirit is riding the cold winds, and the final moment where Varamry is 'seen' - is meant to hint at the idea that there exists a relationship between skinchanging, the weirwoods, and the Others.

Although it's not a new concept within this thread, it is still interesting to see Melisandre explicitly refer to the Nightfort as the "heart of the Wall," since we might assume she's not speaking from a historical perspective--rather, she is probably stating what her own magical intuition is telling her about the nature of the Wall.

I had no idea that he envisioned the Lannister castle as being entirely inside the Rock; I was one of the people that thought it was a castle sitting atop the Rock. I wonder if there's some easy path to get daily sunlight, or if we're supposed to envision the denizens of the Rock as all pale and afflicted with rickets.