[2] According to some historians, in Roman times a military stronghold existed on the current location of the abbey, commanding the main road leading to Gaul from Italy.

The oldest extant account is that of a monk, William, who lived here in the late 11th century and wrote a Chronicon Coenobii Sancti Michaelis de Clusa.

He sets the foundation of the abbey in 966, but, in another passage, the same monk maintains that the construction began under the pontificate of Sylvester II (999–1003).

In addition, it is noted that the cult of St. Michael, the archangel who warred with Lucifer, typically bases its churches on pinnacles or hard-to-reach places, for example, Mont Saint-Michel in France.

Later the abbey developed under the Benedictine rule, with the construction of a separate building with guest rooms for pilgrims following the popular Via Francigena and of a church-monastery (1015–1035), probably on the remains of the ancient Roman castrum.

Abbot Ermengardo (1099–1131) had a new large, 26 m-high basement built from the foot of the hill to its peak, on which a new church (the one existing today) was added, including the surrounding structures.