Mont Saint-Michel is a tidal island and mainland commune in Normandy. The island has held strategic fortifications since ancient times and has been the seat of the monastery from which it draws its name since the 8th century AD. Its structural composition exemplifies the feudal society that constructed it: on top, God (the abbey and monastery); below, the great halls; then stores and housing; and at the bottom, outside the walls, houses for fishermen and farmers. One of the most dangerous aspects of the landmark is the tidal range, which varies by up to 14 meters. The tide comes in with incredible speed—traditionally described as 'at the speed of a galloping horse.' During the French Revolution, the monastery was actually closed and converted into a prison, known as the 'Bastille of the Seas.'