In the 9th century, a castrum, a wooden castle built on a mound of earth, was built to counter the Viking raids that had previously sacked the city of Durivum (Saint-Georges-de-Montaigu). For strategic reasons, this fortress was built on a rocky spur at the junction between the Maine and Asson rivers. Maurice I of Montaigu, the first known lord, witnessed the consecration of the church of Saint-Nicolas de la Chaize-le-Vicomte in 1099. His successor until 1174 was Brient I of Commequiers, married to his daughter, Agathe de Montaigu. Later, his grandchildren, Brient II, Maurice II "the Younger," and Marguerite, succeeded him. It was perhaps this Brient II who had the Romanesque keep built in 1218, a large square tower with flat corner towers. He also removed the wooden palisade, replacing it with a wide moat extending to the Saint-Georges Gate.
The Montagues (also known as Belleville by marriage around 1180) pledged allegiance to the Plantagenets, Henry II, Richard the Lionheart, and John Lackland. Margaret of Belleville-Montaigu, daughter of Brient de Montaigu and Agnes de Belleville, granddaughter of Maurice I de Montaigu, sister of Maurice II de Belleville-Montaigu, married Hugh of Thouars in 1203. In 1227, the couple submitted to Philip Augustus through the ceremony of homage. Hugh of Thouars died in 1229, and Margaret remarried in 1235 to the Duke of Brittany, Mauclerc, whose real name was Pierre de Dreux. Marguerite de Montaigu died in 1241, childless. The lordship of Montaigu fell to her nephew, Maurice III de Belleville-Montaigu (d. c. 1297; son of Maurice II). He became lord of Montaigu upon the death of Mauclerc in 1250[2], and his son Maurice IV (d. c. 1292) ensured the succession through his children Jeanne (d. 1284 Geoffroy II David de La Flocellière) and Maurice V de Belleville-Montaigu (he married 1° Sibylle, daughter of Geoffroy VI de Châteaubriant, then 2° Isabelle/Létice de Parthenay, daughter of Guillaume V Larchevêque de Parthenay). Jeanne de Belleville (c. 1300-c. 1359), daughter of Maurice V of Montaigu and Létice of Parthenay, married Olivier IV of Clisson for the second time in 1328. Olivier IV was executed by beheading after being suspected of "felony" against the English, during the Hundred Years' War. Jeanne became a privateer with her children Olivier V and Jeanne de Clisson, in revenge, and was banished from the kingdom of France. While she attacked French ships, the fortress of Montaigu was held by the English, according to the Treaty of Brétigny-Calais, a garrison of which engaged in pillaging the surrounding area. Jeanne lost her ship and withdrew with her children to England. Olivier V, having grown up, rallied to the king of France, recovered his property in 1362 and fought the English with Bertrand du Guesclin. In 1373, Olivier and Bertrand succeeded in driving the English out of Montaigu, and Olivier ceded the lordship to his sister Jeanne, who had married an English captain, Jean Harpedenne. The latter sided with the King of France in 1373 and became Jean de Belleville. His grandson, Jean III, married Charles VII's half-sister, Marguerite de Valois, in 1433.
Louis I de Belleville succeeded Jean III. King Louis XI advised him to fortify the town between 1464 and 1468 to protect it from the Bretons.