At the end of the 17th century, by the will of Louis XIV, the Fort Royal of Sainte-Marguerite Island became a royal prison and the symbol of the monarch's absolute power. Thick wrought iron gates were added to the windows, and the building housing the cells was transformed into a high-security area with corridors closed by revolving doors making communication impossible. The only possibility of breaking the isolation was the mass celebrated in the chapel located at the end of the prison. The cells were equipped with chimneys and latrines testifying to the importance of the rank of the prisoners who could choose their decoration. The mysterious Man in the Iron Mask spent eleven years in Fort Royal from April 1687 to September 1698.