This is wonderful, and I'm so excited you're continuing.

So, they're battling an avatar of Osiris. Yeah, I can see how this could get epic. *g* Giles noooo! And what the hell has happened to Willow? I call fishy! Your plot is making me think of descant descending; I wonder if you're planning a similar trip to the underworld, or another plane of existence.

I love Buffy's descriptions of the castle -- the chill, the thick walls, the transient bird-song laughter. I like all subtle ways you let the setting reflect the characters' inner selves: strong, but isolated; its defenses are also its weaknesses.