The island boasts a strategic location at the mouth of the Vilaine, and was witness to many sea battles. It now houses two military forts:
- Fort Ré, a semi-circular shape built circa 1756, which served as a home for old guards.
- Fort Carré, known as Vauban's fort built in 1845 in front of Flaubert.
Abandonned by the military a century ago and by its last inhabitants in 1986, it is has now been the property of the Coastal Protection since 1994. Only yachtsmen will have the chance to approach it but you can still admire it from the coast.
