The castle was probably built around 1035-1054 by the Montforts, a powerful family of central Norman lords to defend themselves against their neighbour and to control the road from Pont-Audemer to Brionne, and has seen its role change.
This is in fact a very representative case of one of these attempts of the emancipation of certain lords from central power.
The keep, probably built in c. 1123 by Hugues IV de Montfort, was confiscated in 1124 by Henry I Beauclerc who gave Galeran, count of Meulan charge of it in c. 1128-1129. In 1153, it was taken by force by Robert II de Montfort, son of Hugues, who considered it to be his rightful property. The duke only managed to take it back in 1161, but Robert again tried in vain to take it in c. 1173-1174. Between 1161 and 1197, the castle was held by royal agents who were liable to be dismissed and who were thus docile. It was not until 1197, that a Montfort, Hugues V took charge (rather than the ownership) of the castle and regained the honour of the Montforts. For a short while only, because in 1204, it fell to Philippe Auguste after John Jean Lackland had demolished a large part of it. 
The fortified unit occupies an area of c. 280 m x 165 m (4 ha 63 a) bordering the slopes of the Risle which defends the entire southwest side. The other sides are protected by two courtyards tightly hugging a thick ditched polygonal wall, which is in a considerable state of dilapidation, flanked by six towers of various shapes.
The keep is set in the southwest corner of the perimeter wall. Its most westerly wall housed a chapel dedicated to Saint Nicolas, which had been given to the Abbey of Bec by Robert de Montfort.

Bibliography

- Philippe-Lemaître, Histoire du château et de l’église de Montfort-sur-Rille (Eure), Bulletin monumental, 21, 1885, p. 537-594.
- Fichet de Clairfontaine F., La famille de Montfort-sur-Risle et ses possessions du Xe au XIIIe siècle, Mémoire de maîtrise d’histoire, Université de Caen, ex. dactyl. cons. à la B. U. Caen, 1981-1982, p. 109-113 et 141-143.