About a mile off Marseille sits a three hectare limestone rock crowned by the Château d’If. King Francis I visited around 1516 and ordered a fortress to shield the port, and between 1524 and 1531 builders raised a square, three story stronghold with three cylindrical towers. It looked fearsome enough that Charles V dropped his plans to attack Marseille, yet it never saw battle. When the military engineer Vauban inspected it in 1701, he judged it very badly built, with mortar slapped over natural rock.

A useless fort on an isolated island found other work. Napoleon parked the embalmed body of General Jean-Baptiste Kléber there so no tomb could rally Republican opposition, and the state then turned the rock into a prison ringed by deadly currents. This episode traces that grim history, including more than 3,500 Huguenots held there, and shows how Alexandre Dumas buried it all in 1844 with The Count of Monte Cristo. Roughly 100,000 visitors a year now come to see the cell of Edmond Dantès, a man who never existed.

• Kléber was assassinated in Cairo in 1800, and his coffin sat on the island for 18 years because Napoleon feared a martyr’s grave on the mainland.
• Wealthy inmates such as Mirabeau could rent upper cells called pistoles, with windows and fireplaces, while the poor were chained by the dozen in windowless dungeons cut into the rock.
• No real prisoner is known to have escaped the island and survived. After it opened to the public in 1890, operators hacked a hole between two cells and labeled it the cell of Dantès.
• Mark Twain toured the still working prison in July 1867 and was shown the supposed cell of the Man in the Iron Mask, who was never held there.
• Gaston Crémieux, a leader of the Paris Commune, was shot against the walls in 1871, and the 2002 film of the novel was shot in Malta instead of on the real island.