When I finished reading yesterday, IJ was down; hence the belated comment.
That was awesome! I love how your fic is like the curve of a pendulum, showing first the story of Snape's despair (and tricking the reader, wonderfully done, that) until the point is reached where Snape makes his decision and risks everything. And then it turns, shows what really is going on, Harry's growing despair and his decision to risk everything.
There are many wonderful moments in the second part. Your idea of the castle as a sentient being with the intellect (and emotional capacity) of a five-year-old is fabulous. I love how it releases Snape in a temper tantrum. I especially liked the role of Binns (I don't think I've ever read him in a leading role before), and the showdown in the Great Hall is perfect.
But I have to admit, I like the first part even more. Your Snape, always on the edge, is fascinating. He's like a bomb, threatening to explode every moment, and it's Harry's control that saves him again and again. It's a great touch that the reversal of roles in the bedroom is part of their coping strategy.
How ironic, he thought on the verge of sleep, all those years of begrudgingly protecting him, and now...now that I'd do it gladly, I can't. Ah well, something else to learn to let go of....
I think that was the moment when I fell in love with your Snape ...
I love the Alice references, Snape's tea cup that's waiting for Harry when he returns to the rabbit hole. When it was cold at the end, my heart missed a beat.