General view

This rotunda was built by the Roman emperor Constantine I between 326 and 330, originally as a tomb for himself, but later assigned to his mother, Helena, who died in 328. Because of its status it was built in a privileged position close to the ambulatory of the Basilica di Santi Pietro e Marcellino. This structure is a monumental record of the practice of 'ad sanctos' burial - that is, of burial close to the graves of venerated martyrs.

The building is on the circular plan, and is constituted by two cylinders, the upper one being of smaller diameter (c. 28 m). The original height was c. 25 m, now reduced to c. 18 m.

Internally, the lower cylinder has an octagonal shape. At the vertexes are niches, alternatively rectangular and semicircular; one of them housed the entrance. In correspondence with the niche, in the upper ring, were eight arcaded windows.

The rectangular niche facing the entrance most likely contained the sarcophagus of Helena, in red porphyry, decorated with war scenes, as it was probably originally to be used for Helena's son, the emperor Constantine.

View the ground plan and cross-section of the building.