I've half a mind to print out this story and add it to the end of my copy of DH. What a perfect way to continue the story. You fleshed out all of the characters brilliantly (the addition of the castle itself as a character was genius, by the way); you stayed within the bounds of canon, which I think is always impressive for a slash fic; and you even managed to get Harry and Draco together without it feeling forced or contrived, without turning Ginny into an evil hellbitch, and without piling on the infidelity angst.

In short, that was amazing.