Château d'If is a 16th-century fortress, commissioned by King Francois I, set atop the smallest island in the Frioul archipelago less than a mile from Marseille. It was later used as a prison, considered ideal for its isolated, escape-proof location (similar to San Francisco's Alcatraz). Visitors to Marseille often travel by boat to this beautiful and mysterious monument, especially since it inspired the prison setting of Alexandre Dumas' famous book, "The Count of Monte Cristo." 

Once on the island, visitors are free to wander Château d'If at their leisure. Recent travelers described the experience as interesting, both for the ancient history exhibited here, as well as for the stellar views of the bay and Marseille in the distance. Since the island boasts only one outdoor cafe, some wish that there was more in the way of eateries or gift shops – even as a place to shelter from the cold winds of the island and kill time before boarding the ferry back to the city. 

To reach Château d'If, many travelers hop aboard a Frioul If Express shuttle boat from Marseille's Vieux Port. A round-trip boat ticket costs 10.80 euros (about $12.80) per person, though for families of four or more it costs 8.10 euros (about $9.60) per person. Kids younger than 4 ride for free. Keep in mind that entrance to the fortress costs extra – 6 euros (around $7). Château d'If is open every day except Monday, from Jan. 2 to April 1 and from Oct. 2 to Dec. 31, from 10 a.m. to 5 p.m. From April 2 to Sept. 30, it's open daily from 10 a.m. to 6 p.m. Visitors can also take a 90-minute self-guided tour of Château d'If, or if scheduled in advance, a 30-minute guided tour. If you purchased a City-Pass, your admission is covered. For more information on facilities and services, visit the monument's website.