"The Castle of Dark" (published 1978) is a young adult story by Tanith Lee. A girl Lilune is kept secluded in an old deserted castle by two old wise women. She summons a harper Lir who rescues her, and helps her to see the world beyond the castle. But Lilune is not alone in leaving the castle, the dark has left it too and is following her and Lir . . .
The first time I read this I was struck by the symmetry of the chapters, alternating chapters are told from Lilune's then Lir's viewpoints. Tanith Lee is a great crafter of stories, a true stylist of what she writes. The story is also a page-turner, I didn't know where it was going yet when it got to the resolution it was the right resolution.
This young adult novel is enriched by Tanith Lee's respinning of other mythologies, to create something fresh and new yet archetypal and deep. So at one stage Lilune is cast into a river in a box which makes me think of Osiris. The guardian hags make me think of Macbeth. The harper Lir by name and calling makes me think of Irish legends.