Château d'If was the first royal fortress erected in Marseille in 1529 by order of King Francis I on the strategic island of If in the city's harbor. Built on a square plan with 91-ft long sides (28 m) and flanked by three towers (Saint-Christophe, Saint-Jaume, and Maugovert), the fortress spreads on three levels. Vauban later completed it with low coastal batteries, a guardhouse, and barracks. Used as a state prison for 400 years, Château d'If was made famous by Alexandre Dumas's The Count of Monte Cristo and today welcomes some 100,000 visitors a year. The castle and its escarp wall were listed as Historical Monuments in 1926.