The Castle Mystery — Gertrude Chandler Warner

One warm night four children stood in front of a bakery. No one knew them. No one knew where they had come from.

Henry, Jessie, Violet, and Benny used to live alone in a boxcar. Now they have a home with their grandfather, and they're going to visit a castle!

Built long ago, the castle is now being turned into a museum. As the Boxcar Children help to restore it, they see mysterious lights and hear strange music. Faces appear in windows -- and then disappear. Can the children solve the mystery hidden within the castle's walls?