Along the Via Nomentana, after the crossing with Viale XXI Aprile, is the complex comprising the Church of St. Agnese, the catacombs, a large basilica and the mausoleum of St. Costanza. 

The catacombs developed between the third and fourth century AD on three floors. Here was buried the martyr Agnese, who died between the mid-3rd and the first-4th century. In the 4th century a small basilica was erected on the first floor of the cemetery. 

Constantia, or his father Constantine, erected a huge three-nave basilica near the tomb (337-350 AD), traces of it are still visible. Subsequently, to the south-west of the basilica, she built a circular construction which was to be her mausoleum, Costanza died in 354. 

The mausoleum is one of the most important museums of the late ancient architecture. Is the first example of round building with an ambulatory. The outline of an oval vestibule can still be traced in front of the entrance. The rotunda itself is covered by a dome resting on a drum which is supported on a ring of twinned columns linked by elegant arches. 

The surrounding barrel-vaulted gallery is still adorned with its original fourth century mosaic. The vault is divided into panels and covered with a variety of motifs against a light background: floral and geometric details, portraits in medallions, vine tendrils entwined with harvest scenes. 

The mosaics in the side recesses have Christian themes: God handing down the law to Moses and Christ giving the new law to St Peter and St Paul. In the recess opposite the entrance is a copy of Constanza’s sarcophagus, the original being in the Vatican museums. 

In the 7th century pope Honorius built another basilica upon the tomb of Sant’Agnese: a Christian basilica of byzantine influence as proven by the mosaic in the apse, one of the most beautiful examples of byzantine mosaics in Rome dating back to the first half of the 7th century: St Agnese between pope Symmachus (498-514) and Honorius (625-638), holding a model of the church.