It's position--on an island just a few hundred metres from land--made it accessible at low tide for many pilgrims to its abbey, and defensible as the incoming tide stranded, drove off, or drowned threats on foot. The island remained unconquered during the Hundred Years' War. A small garrison fended off a full attack by the English in 1433. Louis XI recognised the benefits of its natural defence and turned it into a prison, which it regularly served as during the Ancien Régime.
