The Château d'If is a fortress (later a prison) located on the island of If, the smallest island in the Frioul Archipelago situated in the Mediterranean Sea about a mile offshore in the Bay of Marseille. It is famous for being one of the settings of Alexandre Dumas' adventure novel The Count of Monte Cristo (published 1844). The château was built in 1524-1531 on the orders of King Francis I as a defense against attacks from the sea. It served as a prison until it was opened to the public on September 23, 1890.