In the tenth century, Rollo made peace, at Saint-Glair-sur-Epte, with king Charles the Simple. This prince gave him his daughter Giselle and the province of Neustria which he named Normandy.
In 966, Rollo’s grandson, Richard-the-Fearless, established on Mont Saint-Michel the Benedictine monks who then spread across Europe and became the most justly celebrated of the time.
After many vicissitudes, the church started in 1020, was finished in 1135 by Bernard, a professed monk of the abbey of the Bee and a prior of Gremont, who ordered a beautiful, high and strong tower to be built on the four large pillars of the chancel.
After its completion, the church was composed of the chancel, two transepts, and a central nave. The front over the principal one had three doors flanked by large windows.
There was a court-drawn up in front of the foundations built by Hildebert seconded by Robert de Torigni. This marked a period of great prosperity for the Abbey.
During the thirty-two years this abbot governed it, 1154 to 1186, the study of science, literature, and poetry received a considerable impulse. He greatly enlarged the monastery by building the hostelry and the infirmary on the south and on the west, around the Roman works, various buildings joining together the offices from the south to the north, and over which he constructed two towers joined by a porch in front of the romanesque church.