The stairs down to the crypt are in the right side of the Chapel of St Agnes. The crypt is formed from three chambers made from ancient Roman remains of the stadium, one of these being the traditional brothel where St Agnes was abused.

At the foot of the stairs you will see a 17th century fresco of St Agnes Supported by an Angel, and below this is an epigraph of the acta of her martyrdom. Further on, the altar has a marble relief by Giovanni Buratti showing St Agnes Being Led to her Martyrdom. This is a disturbing piece of work, because it depicts an eleven-year-old girl being taken out of the brothel by ancient Roman soldiers and the execution of the scupture is very realistic.

The last room is the ancient oratory which gave rise to the church. It has two columns with Ionic capitals supporting the ceiling vault, and some remnants of Cosmatesque decoration in the floor.