The Château de Beaumont-sur-Oise is an ancient castle, one of the most important in the valley of the Oise. It has a rectangular Romanesque keep with buttresses 25m high and 5m wide. It was probably built by the count Mathieu (1090-1151) to replace a preceding timber structure of the castrum type which had existed from the 3rd century on this rocky outcrop. It was destroyed and rebuilt several times between the 10th and 17th centuries and was no more than a ruin by the 19th century.
The town was developed and built around the castle, with construction in the 10th century of the castle's collegiate church and of a parish church. In 1226, Louis XI became Count of Beaumont and lived in the castle.
The Hundred Years' War and the Wars of Religion were the reason for construction of the upper part of the walls and the keep. The English occupied the town for forty years and the castle itself between 1420 and 1435, when the French artillery of Henri IV caused major damage to the towers of the fortress.
At the time of the French Revolution, the castle was destroyed and then sold off as a national asset. Archaeological excavations carried out from 1984 have revealed the existence of an 11th-century monastery. The castle is owned by the commune.
The Chapel of St. Martin is the only completely preserved Romanesque building in Vyšehrad and one of the oldest in Prague. In was built around 1100 in the eastern part of the fortified outer ward. Between 1100 and 1300, the Rotrunda was surrounded by a cemetery. The building survived the Hussite Wars and was used as the municipal prison of the Town of the Vyšehrad Hill.
During the Thirty Years’ War, it was used as gunpowder storage, from 1700 to 1750, it was renovated and reconsecrated. In 1784, the chapel was closed passed to the military management which kept using it as a warehouseand a cannon-amunition manufacturing facility. In 1841, it was meant to be demolished to give way to the construction of a new road through Vyšehrad. Eventually, only the original western entrance was walled up and replaced with a new one in the sountren side. The dilapidating Rotunda subsequently served as a shelter for the poor.