Costanza, niece of Constantine, had this basilica erected in honor of the martyred St Agnes, one of the most venerated Roman saints, and for the conservation of her remains. The original complex was conceived of as a majestic construction, the traces of which can still be seen from the Piazza Annibalino area. The present church was built in the 7th Century in the Byzantine style. The mosaic in the apse depicts St Agnes with the symbols of martyrdom and the Popes Simmaco and Onorio at the sides, the latter with a model of the church in his hand. One of its many restorations added the bell-tower and the coffered ceiling in gilded wood. Under the high altar is the tomb of the martyrs Agnes and Emerenziana. The former's statue stands above the altar; it was made in 1605, using the torso of an ancient alabaster statue, to which were added the head, hands and the clothes in gilded bronze. The catacombs on three levels, devoid of pictures, were constructed during the time of the Christian persecution.