Fort National sits on a rocky outcrop a few hundred metres off the walled city, accessible on foot at low tide only. Built in 1689 to Vauban's designs, it served as a prison for British captives during the Revolutionary and Napoleonic wars — the prisoners carved graffiti into the walls that is still visible. The fort can be visited during summer months when guided tours are available.
The approach at low tide, walking across the wet sand toward the fort, is one of the memorable experiences of a Saint-Malo visit. The view back toward the ramparts from the fort is also excellent. Check tide times carefully — the causeway floods within minutes of the tide turning, and visitors have been stranded. The tourist office posts a daily tide schedule.