The Treasure in the Royal Tower (novel)

Treasure in the Royal Tower is a young adult novel by Carolyn Keene in the Nancy Drew stories.
Plot summary.
Nancy takes a vacation in Wisconsin when the library of the place she is vacationing at is vandalized. Nancy, along with her friends George Fayne and Bess Marvin, must survive an unknown assailant while discovering the secret passageway's inside of an old castle.
Adaptation.
The fourth installment in the Nancy Drew point-and-click adventure game series by Her Interactive, named "", is loosely based on the novel.