William de Volpiano, the Italian architect who had built the Abbey of Fécamp in Normandy, was chosen as building contractor by Richard II of Normandy in the 11th century. He designed the Romanesque church of the abbey, daringly placing the transept crossing at the top of the mount. Many underground crypts and chapels had to be built to compensate for this weight; these formed the basis for the supportive upward structure that can be seen today. Today Mont-Saint-Michel is seen as a Romanesque style church.
Robert de Thorigny, a great supporter of Henry II of England (who was also Duke of Normandy), reinforced the structure of the buildings and built the main façade of the church in the 12th century. In 1204 the Breton Guy de Thouars, allied to the King of France, undertook the siege of the Mount. After having set fire to the village and having massacred the population, he was obliged to beat a retreat under the powerful walls of the abbey. Unfortunately, the fire which he himself lit extended to the buildings, and the roofs fell prey to the flames. Horrified by the cruelty and the exactions of his Breton ally, Philip Augustusoffered Abbot Jourdain a grant for the construction of a new Gothic-style architectural set which included the addition of the refectory and cloister.
Charles VI is credited with adding major fortifications to the abbey-mount, building towers, successive courtyards and strengthening the ramparts.