Fort Royal stands on Sainte-Marguerite, the largest of the Lérins Islands off Cannes. The island had ancient occupation, and Roman remains, including cisterns and material recovered from nearby shipwrecks, show its place on Mediterranean routes. The present fortress began in the early seventeenth century as France and Spain contested the coast. French construction was followed by Spanish occupation in 1635, when troops strengthened the position with defenses overlooking the channel. French forces recovered the island in 1637 and renamed the stronghold Fort Royal.

The monarchy expanded the fort with walls, barracks, powder magazines, a chapel, and harbor defenses. Engineers associated with the era of Cardinal Richelieu and later Vauban shaped its military form. Yet Fort Royal became famous less for battle than imprisonment. Cells appeared from the 1630s, and in 1685 the Crown made the fort a state prison. Such prisons allowed Louis XIV’s government to confine selected people through royal authority, often without ordinary public trial, while the island’s isolation reduced opportunities for escape or communication.

In 1687 governor Bénigne de Saint-Mars arrived with a closely guarded anonymous prisoner later called the Man in the Iron Mask. Contemporary evidence indicates that the prisoner usually wore a dark cloth or velvet covering rather than a permanent iron mask. He remained at Fort Royal until 1698, when Saint-Mars transferred him to the Bastille; he died there in 1703. His identity has never been conclusively established. Later writers transformed secrecy into legend, proposing royal twins, disgraced ministers, and many other candidates.

Other prisoners reveal the fort’s broader history. Protestant pastors were held after Louis XIV revoked the Edict of Nantes in 1685. During the nineteenth century, captives connected with French conquest and resistance in Algeria were confined on the island, and a Muslim cemetery preserves evidence of deaths in detention. Marshal François Bazaine, convicted after the Franco-Prussian War, escaped from the fortress in 1874. Military prisoners and political detainees therefore occupied the same walls across changing regimes, from Bourbon monarchy to republic.

The fort later served military, educational, and museum purposes. Its archaeology collections developed from excavations and underwater finds around the islands, while former cells became exhibition spaces. The institution, once called the Musée de la Mer, is now the Musée du Masque de fer et du Fort Royal. Preservation must balance the famous masked-prisoner story with the fort’s ancient, military, religious, colonial, and carceral histories. The picturesque crossing from Cannes can conceal that this was a landscape of coercion. Its walls document Mediterranean rivalry, absolute monarchy, persecution of Huguenots, France’s Algerian empire, military justice, and the continuing power of historical mystery.

The island’s history is therefore larger than the mystery attached to its most famous prisoner. Fort Royal connected royal engineering, Mediterranean warfare, forced confinement, religious persecution, colonial expansion, and the policing of political opponents. Archaeological work and restoration have revealed successive defensive phases and everyday spaces that written accounts overlook. Museum interpretation now places the prison within the wider Îles de Lérins landscape, including the neighboring monastic island of Saint-Honorat and the strategic roadstead off Cannes. Visitors encounter a picturesque coast, but the surviving cells and ramparts demonstrate how an apparently isolated island could be integrated into centralized systems of surveillance and military control for centuries.