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.
The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.