The Castle Conundrum — Franklin W. Dixon

AN ANCIENT HIDDEN TREASURE STIRS UP A CUNNING PLOT!
Teen Village International brings Frank and Joe to the south of France to rebuild a deserted town. The finished project will be a center for refugee children. But word is that the local castle is haunted -- by the centuries-old ghost of its lord, who hid a fortune in diamonds from bandits within its thick stone walls.
Spooky lights, strange noises, and several close shaves convince the Hardys that someone is trying to sabotage the project. A pair of French twins, descendants of the owner, want to save the castle from developers. Every facet of the case leads to a dead end -- and the Hardys are hunting a desperate enemy!