The clue in the crumbling wall - Carolyn Keene

When Nancy is asked to find a professional dancer who disappeared several years before, the young detective becomes involved in a mystery reaching far beyond a missing-person's case. During Nancy's investigation at Heath Castle, she and her friends Bess and George realize that its crumbling walls contain a secret, but what is it? And who are their enemies that try to foil their every attempt to unravel the intricate puzzle? Danger lurks in a castle tower and throughout the vine-tangled grounds of the estate. The girls' gripping adventures culminate in a dramatic climax when Nancy exposes a sinister plot to defraud the dancer of her inheritance.