The Benedicine settlement, which was founded in the 6th-7th centuries, on a wwooed island embraced by the river and protected by sea, after the year 1000 became an inportant monastic community. Those who stayed there included Guido d'Arezzo, the monk who invented musical notation. To the side of the Byzantine-Ravenna style church, stands a tall bell tower built in 1063, while the inside shows a 14th century cycle of frescoes of the Giotto school, and a wonderful mosaic floor.