This is so fabulous! Long and intense and detailed and surprising. Until Severus and Harry were talking about the fact, I had no idea that nobody else knew--or could know--Severus was there And even better I had no idea there was a secret being kept from me; the narrative didn't feel forced or skewed in any way, which is quite an accomplishment in a story that depends for so much of its impact on a secret.
But not all its impact. I'm pretty sure I would still have been on the edge of my seat even if *everybody* had known Snape was stuck in his rooms, because you made his--and Harry's--daily struggle so real, and so gripping, so hopeful and so desperate. I loved the interaction between them, how you could see the relationship develop through the flashbacks and the conversations and the day-to-day living. Severus' wavering between determination and resignation and despair was so painful, but so completely how I could see him reacting to the situation. Harry's final solution to the problem was perfectly in character, too--very Gryffindor, but more than that an exemplary portrayal of his loyalty to those he loves.
I really liked Minerva here (I have a soft spot for her, anyway), and Hermione and Ron, the way they kept trying--gently and firmly and lovingly--to get Harry to come out, to go on holiday, to live his life, and I *love* thinking about how they're going to react when they find out what was really going on all this time; beyond my joy for Harry and Severus themselves, that's part of what makes the ending so sweet. Mostly, though, I love it because there is so much *possibility* in it, so much hope and wide-open futurity--just the way their lives should have been in the aftermath of Voldemort, but even better now because they're together.