You know that quip that gothic novels are primarily romances about houses? That's pretty much Mary Stewart to a tee. Which is why the Arthurian stuff isn't her most exciting, because there are obviously not great old English country houses in that period. I find it hard to remember which of her books is which cos they are kind of samey. I think the one you read is probably Touch not the cat. Airs above the ground has dancing Lipizzaners in it, in addition to the usual elements. The ivy tree is one of those classic impostor gothics, a bit like Josephine Tey. Also her YA / chapter book Ludo and the star horse is just gorgeous.