The Catholic faithful earn indulgences for visits to this major basilica. Along with the Lateran palace, it was the site of the original papal headquarters until the move across the river to St Peter's and the Vatican in the 14th century. Constantine's second wife, Fausta, gave the plot of land to Pope Melchiades to build the papal residence and church in 313. There are few traces of the of the original basilica, which was done in by fire, earthquake and barbarians. It has been heavily restored and rebuilt: the end result is a vast, impersonal, over-decorated hangar.
The façade with its 15 huge statues of Christ, the two Johns (Baptist and Evangelist) and 12 Doctors of the Church, is part of the 1735 rebuilding by Alessandro Galilei. The interior bears the stamp of Borromini, who transformed it in 1646; for centuries he was derided for encasing the original columns in stucco, though experts now believe that the ancient supports had been replaced by nondescript ones in the 15th century. A few treasures from earlier times survive: a much restored 13th-century mosaic in the apse, a fragment of a fresco attributed to Giotto (hidden behind the first column on the right) showing Pope Boniface VIII announcing the first Holy Year in 1300, and the Gothic baldacchino over the main altar.
Off the left aisle is the 13th-century cloister, with delicate twisted columns and fine cosmatesque work by the Vassalletto family. A small museum off the cloister contains papal vestments and some original manuscripts of music by Palestrina.
The north façade was designed in 1586 by Domenico Fontana, who also placed Rome's tallest Egyptian obelisk out front. This was part of Pope Sixtus V's 16th-century urban renewal scheme. Also on this side is the octagonal baptistry that Constantine had built. The four chapels surrounding the font have mosaics from the fifth and seventh centuries, and bronze doors said to come from the Baths of Caracalla.