Château de Montfort, Medieval castle in Crolles, France.

Château de Montfort, also called château Robert locally, is a ruined stone castle on a slope below the Chartreuse cliffs, above the hamlet of Montfort and the Grésivaudan valley. Its plan forms a flattened triangle or half-moon: a curtain wall of just under 100 metres links a guard post and a kitchen, with dividing walls enclosing inner courtyards. The keep has disappeared; a tall flat-faced wall with a small grilled window and a curved terrace wall remain.

Montis Fortis is first mentioned in 1069 in the cartulary of Domène, probably as a castral motte on the promontory. The stone castle was built in the 13th century by the dauphins of Viennois, and Béatrice de Faucigny stayed there in the early 14th century. A 1339 enquiry for Humbert II describes a 47-toise circular enclosure, a 28-metre square tower and a kitchen; the castle passed to Amblard de Beaumont in 1343 and was already ruined when his family sold it in 1617.

The castle served as a seat of local nobility and remains a defining landmark for the village today. Its ruins continue to shape how residents and visitors understand this area's historical importance.

The ruins are not open to the public and a safety zone has been set up because of falling stones and unstable ground, so they are viewed from outside. A small marked path along the funicular track and the Montfort stream leads up from the hamlet of Montfort. Bus line G3 and the Isère Express 1 stop at Montfort; the nearest station is Brignoud, about 5 km away.

The 1339 enquiry for Dauphin Humbert II records a circular enclosure 47 toises long, a square tower 28 metres high with four floors, and a kitchen at one end of the flattened triangle. The keep itself has vanished; the ruins now link the guard post and the kitchen.