The Château d'If sits on a small limestone island just over a kilometre off Marseille, the closest of the four Frioul islands. François I had it built between 1524 and 1531 as a coastal defence, but it never fired a shot in anger. Instead it became a prison, holding Protestants during the Wars of Religion and political opponents through the 18th and 19th centuries. Among its real inmates were the philosopher Mirabeau and the Communard Gaston Crémieux.
Most visitors come because of Alexandre Dumas's 1844 novel The Count of Monte Cristo, in which the wronged sailor Edmond Dantès is held here for 14 years before escaping. The interior is bare — small stone cells, narrow stairways, a central courtyard — but the rooftop ramparts give a fine view back across to Marseille and out to the open sea. Boats run from the Vieux-Port year-round, usually combined with the larger Frioul islands. Allow half a day with the boat ride. The crossing can be choppy in the Mistral.