I was part of the production team in the Spring of 1976 and spent about three month on the small Italian Island of Elba, you will find production pictures at the exact Google Earth Location of the movie set at 42°43'14.21"N 10°22'35.57"E. The production costs exceeded of what was originally estimated. We had technical problems with some boats and to review the takes the "dailies" had to be shipped to Rome and back. Sadly, the movie had distribution problems and copies are hard to find.
Locals from Capoliveri were hired to build the Citadel out of wood, paper and paint on the base of an old destroyed mine near Innamorata on Elba Island. They had to be driven daily to the movie set about 30 minutes away. Here some private tidbits: After the production Andras Friscay married Jeanette Muhlmann. Sadly, Antonia Reininghaus committed suicide in 2006. She had a short liaison with actor Jürgen Prochnow (Das Boot, IMDb: tt0082096) in 1980. Both had a daughter named Johanna. Tragically her first suicide attempt in 1987 ended with the death of her daughter.