The Emperor Constantine converted part of the Sessoian Palace into a church to house the mother's relics. According to tradition, Constantine's mother, Helen, went on a pilgrimage to Jerusalem, and when she returned back she took back a fragment of the True Cross: this event contributed to render the church a centre of pilgrimage. In the 12C, Pope Lucius II (1144-45) built a campanile without altering the outside wall.