Designed by famed military architect Sébastien de Vauban, construction of this fortress began in 1689. You can access it by walking 300m (984 ft.) over sand at low tide (heed the tidal information, or you may find yourself wading back). Thirty-five-minute guided tours take you into the dungeon and explain the fort’s history; it’s equally rewarding to wander on your own and enjoy the views of the bay.