The mount gained strategic significance in 933 when William "Long Sword", William I, Duke of Normandy, annexed the Cotentin Peninsula, definitively placing the mount in Normandy. It is depicted in the Bayeux Tapestry which commemorates the 1066 Norman conquest of England. Norman Ducal patronage financed the spectacular Norman architecture of the abbey in subsequent centuries.
In 1067, the monastery of Mont-Saint-Michel gave its support to duke William of Normandy in his claim to the throne of England. It was rewarded with properties and grounds on the English side of the Channel, including a small island located to the west of Cornwall, which was modeled after the Mount, and became a Norman priory named St Michael's Mount of Penzance.
During the Hundred Years' War, the English made repeated assaults on the island, but were unable to seize it due to the abbey's improved fortifications.
When Louis XI of France founded the Order of Saint Michael in 1469, he intended that the abbey church of Mont Saint-Michel be the chapel for the Order, but because of its great distance from Paris, his intention could never be realized.
The wealth and influence of the abbey extended to many daughter foundations, however, its popularity and prestige as a centre of pilgrimage waned with the Reformation, and by the French Revolution there were scarcely any monks in residence.
The abbey was closed and converted into a prison, initially to hold clerical opponents of the republican régime. High-profile political prisoners followed, but by 1836, influential figures – including Victor Hugo had launched a campaign to restore what was seen as a national architectural treasure. The prison was closed in 1863 and the mount was declared a historic monument in 1874. The Mont-Saint-Michel and its bay were added to the UNESCO list of World Heritage Sites in 1979 with criteria such as cultural, historical, and architectural significance as well as human-created and natural beauty.
