This is the cathedral of Rome, not St. Peter's! And beside this, there are many reasons for a visit. The splendid floor in Cosmati of the middle of the 1400. The richly decorated coffered ceiling of the middle of the following century. The elegant baroque architectonical interior restructured by Borromimi for the Jubilee of the 1650. The monumental 1300's canopy crowing the papal altar made by Giovanni di Stefano. The late 1500 gorgeous transept made by Giacomo della Porta and decorated with a cycle of frescoes by Cavalier d'Arpino. The glamorous mosaic decoration of the apse that (although restored) dates back to the late 1200 (by Jacopo Torriti). And this is just for some of the artistic values of this church which is also foundamental in the history of the Christianity being the first papal residence and being the seat of the Pope's Vicar for Rome today. The edifice across the street preserves the world-famous Holy Staircase.