Santa Maria del Popolo
The architect or architects of this innovative project remain unknown due to a lack of contemporary sources. Giorgio Vasari in his "Lives" attributed all the important papal projects in Rome during Sixtus IV to a Florentine, Baccio Pontelli including the basilica and monastery of Santa Maria del Popolo. Modern researchers deemed this claim highly dubious and proposed other names among them Andrea Bregno, a Lombard sculptor and architect whose workshop certainly received important commissions in the basilica. The fundamental differences between the façade and the interior suggest that maybe more than one architect was working on the building.
Pope Gregory IV
Gregory also 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.