Castle Merlin, by Ursula Moray Williams (1971).
Susie had her heart set on an organized holiday for kids at Castle Merlin, up near Hadrian's Wall (aside--I myself wouldn't necessarily want to go on holiday to the north of England in January with a bunch of strange kids, but Susie does). Flu threatens to derail her plans, but she feels recovered enough to go, and whines and frets her parent's resistance down (aside-she knows she's been an unsympathetic character, and so does the reader...although all of us who have been 11 ourselves can relate to wanting something so very very badly). So she sets off on a train from London to Castle Merlin; she'll be a day late, but that's no big matter. On the train she meets Bryan, also making his way to Castle Merlin; he's not deeply sympathetic either, but the two form a bond of shared experience.
And then they arrive, and find that the Holiday at Castle Merlin has a falconry theme, in honor of its long ago chatelaine, Dame Alys, famous as an expert on merlins. Though somewhat taken aback (it's all falcons all the time, even though there's only one real falcon around, Guinevere,who belongs to a visiting author of children's books), Susie gets into the falconry spirit, and is very glad she came.
But the past at Castle Merlin has left strong impressions that color the present, and Susie can't for the first few days distinguish what is past from what is present. Bryan takes her down to the dungeon, where a prisoner is being held in miserable conditions (starved and shackled to the wall), and both of them believe he is real....a strange girl shows up in her room, and then is never seen again, and hawks from the past, and Dame Alys herself, still inhabit the castle and its grounds. When Guin the (modern) hawk, Bryon, and Dame Alys valuable book about merlins all go missing on the same day, Susie must figure out how to separate past from present, and figure out how to help Bryan, whose experience with the ghost prison in the dungeons has forced him to confront his own troubles....
Castle Merlin is one of those time slip books that teeters between ghost story and
time travel. Mostly it's ghosts, but there are a few moments when Susie sees physical manifestations of the past that aren't dead
people (like entering a building that's a chicken house in the present
and finding it full of hawks, or wandering in the woods and finding a loose merlin from the past). So I'm going with time slip.
This is a book that will delight anyone who thinks it would be great to spend a week in a castle in winter with a bunch of strangers engaging in group activities related to falcons. I feel I would have been lots more keen on the idea when I was Susie's age, but that ship has kind of sailed for me now. Still I managed to channel my 11 year old self enough to enjoy the book (some of the ghost/timeslipping was really nicely done, with the exception perhaps of the miserable prisoner, which required considerable suspension of disbelief), though I couldn't quite love it.