The dome of the rotunda, Château des Agoult in Simiane-la-Rotonde, France.

The stone ribs descend from the oculus with a slightly helical twist. The oculus at the central peak of the dome was for light and fresh air and, in addition there are four air vents at the base of the dome.

The architecture is Romanesque, 12th century, and was a ceremonial and reception room. Rotunda describes a building with a circular ground plan. It forms part of a castle that sits at the summit of a steep hillside covered in the ancient village of Simiane-la-Rotonde.