What a delicious story! I particularly liked the relationship of Binns and the sentient castle--I found that a really different take on the ghost.
The slow building of suspense, combined with the parcelling out of background details kept me enraptured from the very beginning. I didn't realize until you told me that Harry couldn't speak about Severus--I thought he was just trying to hide it from his friends! I can imagine how worried everyone one was with what they viewed as his instrospection and isolation--and found it particularly poignant that yet again, Harry's circumstances kept him from having a 'normal' life, poor thing! He still felt as if no one really saw the real him.
I thought you handled the revelation of the mystery magnificently, and still managed to keep up (and ramp up!) the angst factor. Your portrayal of a beautifully mature Harry was well done, as were the psychological aspects of Severus' imprisonment. The phoenix/dragon debate was also a nice touch!