The Church was built on part of the barracks of the Roman Castra Peregrina, at a mithraeum dating back to 180 and brought to light in 1973-1975. The building was commissioned by Pope Leo I. The building was circular, originally made up of three concentric circles: a central space bounded by a circle of 22 columns, which a drum; this central part was surrounded by two ambulacra lower ring, of which, the most interior, delimited by a second circle of columns connected by arches, now inserted in a continuous wall, and the outermost (disappeared), closed by a bottom wall. From the corridors, which was accessed from outside by eight small doors, one passed environments radial Greek cross, and from there to the space station, sometimes covered with self-supporting. The interior was richly decorated with marble slabs. In the central space was an enclosed altar. Between 523 and 529, the Church was decorated with mosaics and lined with precious marble. The apse was decorated with a mosaic on gold, on which are depicted the two saints on either side of a jeweled cross, surmounted by a medallion with the bust of Christ on the starry sky, with the hand of God who offers the martyrdom crown. The mosaic, one of the few best preserved examples of this era in Rome, was probably executed by an artist of Byzantine origin. The Church declined in later centuries and lost the original covers. Since 1958 it is under archaeological excavations and restorations.