Château de Saint-Béat dates from the 12th century. It was enlarged by Henri IV (1553 – 1610). Rulers rarely lived in Saint-Béat; the castle was occupied by captains until the 16th century. In 1588, the Parlement of Toulouse passed a law that required the inhabitants of Melles, Argut and Arlos by turns to guard the castle, subject to a fine of 500 écus. The castle never had to repel invasions, though its strategic position close to the Spanish border led to it being described as 'la clef de France' (the key to France).
The castle was surrounded by two enceintes. The keep measures 5m square and had two storeys. The castle provides views over the village and the Garonne valley.
References: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.