Gregory contributed to the architectural development of Rome. In 833, Gregory completely rebuilt St Mark’s Basilica in Rome, adorning the walls with Byzantine-style mosaics, as well as a number of other churches which he either repaired or rebuilt. He rebuilt the atrium of St. Peter’s Basilica, and within the newly decorated chapel within the basilica he transferred the body of Saint Gregory, and from the Catacombs of Rome, he moved Saint Sebastian, Saint Tiburtius, and Saint Gorgonius. He raised the altar in the church of Santa Maria in Trastevere, and founded a monastery close to the church.