Château de Beaumont-sur-Oise, Medieval castle in Beaumont-sur-Oise, France.

The Château de Beaumont-sur-Oise is a ruined medieval fortress on a wooded plateau above the Oise, 20 metres below. Its surviving enceinte, about 200 metres long, 2.40 metres thick and 10 to 15 metres high, forms half of a fourteen-sided polygon with seven round turrets; the entrance had a drawbridge and a vaulted gate. The rectangular Romanesque keep, 25 metres high and 5 metres wide, held three floors of lord's apartments.

The castle is first mentioned in 953, when it was a timber structure on a motte. In the 12th century Count Mathieu replaced it with a stone keep about 25 metres high. The English occupied the castle from 1420 to 1435, and in 1432 the Duke of Bedford ordered its demolition; La Hire rebuilt it in 1434, but John Talbot soon demolished it again. In 1590, during the siege of Paris, Huguenot artillery damaged the towers before Jean de Poutrincourt surrendered.

The collegiate church founded in the 10th century still stands beside the castle ruins, while the parish church and the priory of Saint-Léonor, whose relics of the Irish-rite monk Leonor arrived in the 10th century, have disappeared. Excavations from 1984 to 1987 uncovered the 11th-century priory church, 32 m long and 20 m wide, with its 12th-century cloister.

The ruins stand on a wooded plateau corner above the Oise, about 20 m below; the site is owned by the commune and the archaeological area was listed in 1999. The enceinte measured 200 m long and 2.40 m thick, so allow time to follow the surviving stretches of wall and the round tower bases. The walls were cleared and restored in 1997.

Beneath the castle grounds lie the remains of an early medieval monastery that predated the stone fortification. These archaeological discoveries reveal that the site had religious origins before it became a military stronghold.