The Château d'If (French pronunciation: [ʃɑto dif]; Occitan: Castèl d'If) is a fortress located on the Île d'If (Ilha d'If), the smallest island in the Frioul archipelago, situated about 1.5 kilometres (7⁄8 mile) offshore from Marseille in southeastern France. Built in the 16th century, it later served as a prison until the end of the 19th century. The fortress was demilitarized and opened to the public in 1890.

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