In the 9th century, a wooden castrum was built on a rocky promontory to fend off Viking raids near Durivum (Saint-Georges-de-Montaigu). Later, a stone keep was added with strategic upgrades, including a drawbridge. The Montaigu family, later known as Belleville, pledged loyalty to the Plantagenets. Over time, the property changed hands through marriages and alliances, including a notable period of privateering by Jeanne de Belleville. The fortifications of Château de Montaigu were enhanced in the 15th century under Louis XI's guidance to protect against Breton threats.
Some visible remnants still exist. Originally, there was an oval-shaped enclosure with about ten towers and an entrance gatehouse located in the northeast. The fortified town was situated to the north of the fortress and extended slightly to the east. A dry moat separated the fortified town from the fortress. A barbican protected the entrance gatehouse. A Romanesque keep of the Niort type, with its courtyard and a small enclosure, was located in the castle's courtyard, roughly in the middle in the north-south direction and facing west.
Saint-Georges de Boscherville Abbey is a former Benedictine abbey. It was founded in about 1113 by Guillaume de Tancarville on the site of an earlier establishment of secular canons and settled by monks from the Abbey of Saint-Evroul. The abbey church made of Caumont stone was erected from 1113 to 1140. The Norman builders aimed to have very well-lit naves and they did this by means of tall, large windows, initially made possible by a wooden ceiling, which prevented uplift, although this was replaced by a Gothic vault in the 13th century. The chapter room was built after the abbey church and dates from the last quarter of the 12th century.
The arrival of the Maurist monks in 1659, after the disasters of the Wars of Religion, helped to get the abbey back on a firmer spiritual, architectural and economic footing. They erected a large monastic building one wing of which fitted tightly around the chapter house (which was otherwise left as it was).