This was the first paleochristian church in the city. It was built against the medieval city walls and in a relevant position for the town. It must have been in visual contact before the construction of the borough. Built at the end of the 5th century, it has a circular floor plan. Its central part is on a higher level and it is covered by a tent-shaped roof. The various parts added over the centuries have been removed and only the Gothic central portal from the 14th century has been spared. An exedra of cypress trees flanks the temple and surrounds the lawn on which a column brought from the Sopramuro Square stands out. The interior is structured like that of the Chiesa di S. Stefano Rotondo (Church of Saint Stephen in the Round) in Rome, that is a circular ring with a visible roof; in the middle there is a drum that rests on 16 ancient columns with various shafts, differing in height and building material (granite, cipollino and black marble) surmounted by reused Roman capitals, as well as Corinthian and figurative ones, of various types and dating; the roof is supported by eight arches carried by walled small pensile columns, added in the 14th century.
Interior: the original floor plan of the church had the shape of a Greek cross, with a lantern and 4 chapels in the extensions of the sides (now only the one situated in the apse (known as the crucifix) and the one in front of the entrance are still visible. Going round the ambulatory, from the right: fresco from the 14th century; small baptistery with votive frescoes from the 15th century. Next is the Cappella del Crocifisso (Chapel of the Crucifix) – it has an internal circular floor plan and a polygonal exterior one; the Cappella dell'Angelo (Chapel of the Angel) is a little further on, restored in the ancient structures; going on, there is a Roman cippus with a dedication from the time of Marcus Aurelius and a fresco representing the Madonna del Verde (Madonna of the Green) probably from the Siena school dating back to the end of the 14th century over it; the meaning of this title is unknown, but records show that there had been another venerated image in the Chiesa di S. Lorenzo (Church of Saint Lawrence) in the 10th century, which had the same name – it doesn’t exist anymore. The altar, found at the center of the church, consists of an ancient marble slab over the trunk of a column.