Basilica di Sant'Agnese Fuori le Mura and Mausoleo di Santa Costanza
The lower part of the apse is covered in marble and porphyry. The Byzantine-style mosaic in the upper part is from c. 625, and depicts St Agnes being given the crown of martyrdom from the hand of God. She is dressed as an Augusta, a Byzantine empress - according to legend she appeared like this eight days after her death, holding a white lamb. She is flanked by Pope Honorius, holding a model of the church, and Pope St Symmachus.
This is the catacombs below the church where early Christians were buried.