Château de Fallavier, Medieval castle ruins in Saint Quentin-Fallavier, France.

Château de Fallavier is a ruined castle set on Relong hill in Saint-Quentin-Fallavier, in the Isère department. The round keep is the tallest surviving element of the site, surrounded by the remains of walls and defensive structures on the slopes below.

The castle existed before 1250 and was enlarged and reinforced during the 13th century by the counts of Savoy. In 1360 it was acquired by the king of France, who ceded it in 1369 to Jean, count of Geneva; it later passed to the house of Thoire-Villars and was retaken by the king from Odon de Villars in 1406. In 1430 it was taken by the prince of Orange, and by the 17th century it lay more or less in ruins.

The castle grounds served as an outdoor set for Alexandre Astier's series Kaamelott, including the short film Dies irae that served as its pilot. Volunteers of the association Chantier Histoire et Architecture Médiévales carried out restoration work there between 2002 and 2007.

The site is open during a limited season, and access to the keep involves steep and uneven paths up the hill. Sturdy footwear is a good idea, as some sections require careful footing.

Inside the keep, a narrow staircase cut directly into the stone walls winds its way to the top of the tower. Standing there, it becomes easy to understand why this hilltop position was chosen to watch over the surrounding land.