The castle as a sentient (but almost alien) entity, Binns "fleshed out" (so to speak), the slow ebb and flow of Harry and Snape's developing relationship: all these are wonderful. But what I think I might take with me in the end is the way in which this whole story can be read as an extended (hopeful) metaphor for breaking free from the cycle of depression. So well done...I'm definitely adding this to my recs page when I next update!