I absolutely LOVE how you blended canon into this AU! Since HBP, I've been partial to the idea that the castle was sentient in some way - I remember thinking that if it were possible to turn the castle into a horcrux, well, besides being pretty cool, it was also be incredibly powerful - and I love how it became a character in its own right.
This was a fascinating story! I loved how you wrote both Snape and Harry - they were believable and totally suited the altered canon, and Snape's dialogue and mannerisms were really well done. You made me like the bastard! :-P Definitely an enjoyable read!