In 939, in the church space of Sant Sebastià del Sull there was a small male monastic community made up of six monks and other people. During the 10th century the counts of Cerdanya began making donations to the monastery, but by the 11th century there were no more monks. It is a church dating to the 9th-10th centuries and stood out for its round floor plan, seven and a half meters in diameter, and its walls 1,40 meters thick. The false vaulted roof probably collapsed because the foundation of the church floor was not strong enough. Inside, a bench that was probably the only seat, surrounded the apse and the roundabout. Although the cemetery was outside the walls, burials were found for infants and an elderly adult inside the roundabout. This church possibly served as a model for other temples with a round floor plan built during the eleventh century along the Llobregat.