The small tidal island just to the north of the castle of St Malo was an obvious candidate for fortification, extending the range of the defenders' guns over the deep water where an attacking fleet could come close to the town. In this regard the guns from this fort worked in concert with those in the Fort du Petit Bé and the Fort du Grand Bé to the west. In 1689 Vauban proposed a roughly square fort for the island, leaving the detailed work for Siméon Garangeau, the Director of fortifications at St Malo. It was known as the Fort Royal or sometimes the Fort de l'Islet.
The fort took the form of a square with two demi-bastions and a demi-lune on the landward side, which was also protected by an outer loopholed wall. Following the English naval attacks of 1693 and 1695, the engineer Siméon Garangeau modified the north side of the fort. He added a rounded battery on a lower level where heavier ordinance was mounted. This lower battery had no embrasures so that the guns had an unrestricted arc of fire. This made it possible to track moving targets, such as enemy ships, with ease.
The fort was the largest of the three island forts immediately to the north of the town. There was large barrack building where soldiers were accommodated inside the fort. A large cistern was used to collect rainwater for the garrison. During the French Revolution the name Fort Royal became unacceptable, so it was changed to Fort National, the name it has retained to this day.
Visiting Fort National
Fort National has survived intact, despite the barrack building being damaged in the Second World War. The fort has been restored and is open to visitors every day in the summer and on school holidays. It is possible to walk to the fort at low tide and there is a guided tour lasting half an hour.