Chateau d'If is a fortress located on a small island called If, just off the coast of Marseille, France. The island was named after the Greek word "eidos" which means shape or form. The Chateau d'If was originally built in the 16th century as a prison to keep political prisoners and religious dissidents.

It is best known as the setting of the classic novel, "The Count of Monte Cristo" by Alexandre Dumas. The book features the story of the wrongfully imprisoned Edmond Dantès, who escapes from the Chateau d'If after years of imprisonment.

Today, the Chateau d'If serves as a museum and tourist attraction. Visitors can tour the cells and ramparts, and admire the beautiful views of the Mediterranean Sea from the ramparts. The Chateau d'If is accessible by boat from Marseille and is a popular destination for history and literary buffs alike.