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:Carlsten is a stone fortress built on the orders of King Carl X of Sweden following the Treaty of Roskilde, 1658 to protect the newly acquired province of Bohuslän from hostile attacks. The site of Marstrand was chosen because of its location and its access to an ice free port. Initially a square stone tower was constructed, but by 1680 it was reconstructed and replaced by a round shaped tower. Successive additions to the fortress were carried out, by the inmates sentenced to hard labour, until 1860 when it was reported finished. The fortress was decommissioned as a permanent defense installation in 1882, but remained in military use until the early 1990s.
The fortress was attacked and sieged twice falling into enemy hands. In 1677 it was conquered by Ulrik Frederick Gyldenløve, the Danish military commander in Norway and in 1719 by the Norwegian Vice-Admiral Tordenskjold. At both occasions the fortress was returned to Swedish control through negotiations and treaties.