a fortress (later a prison) located on the island of If, the smallest island in the Frioul archipelago situated in the Mediterranean Sea about 1.5 kilometres (7⁄8 mile) offshore in the Bay of Marseille in southeastern France. It is famous for being one of the settings of Alexandre Dumas' adventure novel The Count of Monte Cristo. "If" is the French word for the yew tree.
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Why, look at one of them prisoners in the bottom dungeon of the Castle Deef, in the harbor of Marseilles, that dug himself out that way;
Mark Twain. The Adventures of Huckleberry Finn
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One day [Jefferson] took a boat to the island fortress of the Château d'If, which would feature so prominently in Alexander Dumas's 1844 novel The Count of Monte Cristo.
Thomas J. Craughwell. Thomas Jefferson's Crème Brûlée, p.92 (2012)