Le Mont Saint Michel
Its unique position of being an island only 600 metres from land made it readily accessible on low tide to the many pilgrims to its abbey. Equally, this position made it readily defensible as an incoming tide stranded, or drowned, would-be assailants. By capitalising on this natural defence, the Mont remained unconquered during the Hundred Years' War with a small garrison successfully defending it against a full attack by the English in 1433. The reverse benefits of its natural defence was not lost on Louis XI who turned The Mont into a state prison and thereafter the abbey started to be used more regularly as a jail during the Ancien Régime from the sixteenth century.