The castle and its ramparts, the remains of the medieval enclosure, dominate the approaches of the Oise and discover a splendid view of the church Saint-Laurent. The Hundred Years' War defeated the upper part of the walls and dungeon. Archaeological excavations conducted in 1984 made it possible to find the site of the walls, rehabilitated in 1997.
At the beginning of the 10th century, Breton monks and clerics left their establishments under the pressure of Norman incursions. Taking with them the relics of their holy founder, Saint Léonor, they headed for the banks of the Oise and settled on the site of the castle. Between 965 and 985, the lords of Beaumont appeared.
The twelfth century marks a first turning point in the evolution of the castle. It was at this time that the construction of its large stone keep (37 m high) began. This keep, higher than the bell tower of the Saint-Laurent church (32 m) was used to store food on the ground floor, and to shelter the lord’s apartments on the floor. In 1223 the last count of Beaumont died, who died without descendants, the castle was then bought by King Louis VIII, passing thus in the royal domain.
In 1226, Louis IX future Saint Louis, resides at the castle and built the Abbey of Royaumont.
During the fourteenth century and the fifteenth century, the castle underwent numerous assaults, numerous destructions and reconstructions: Hundred Years War, sieges of the English, French, Burgundians and Armagnacs. The castle and the town were occupied by the English between 1420 and 1435. The 18th century saw the birth of the last major transformation of the castle. With the birth of the artillery, the weapons are more and more powerful, so it is necessary to strengthen the defensive structure of the castle. The height of the keep will be reduced from 37 meters to about 5 meters. The castle becomes an artillery stronghold.
Then, the castle loses its grandeur and in 1815, it is abandoned. Emile Zola discovers the city and places there the action of his novel The Dream.