The church is the oldest and highest ranking of the four major papal basilicas as well as one of the Seven Pilgrim Churches of Rome, holding the unique title of "archbasilica". Founded in 324, it is the oldest public church in the city of Rome, and the oldest basilica of the Western world. It houses the cathedra of the Roman bishop, and has the title of ecumenical mother church of the Catholic faithful. The building deteriorated during the Middle Ages and was badly damaged by two fires in the 14th century. It was rebuilt in the late 16th century during the reign of Pope Sixtus V. The new structure's interior was renovated in the late 17th century, and its façade was completed in 1735 under Pope Clement XII.

The archbasilica stands over the remains of the Castra Nova equitum singularium, the "New Fort of the Roman imperial cavalry bodyguards". The fort was established by Septimius Severus in AD 193. Following the victory of Emperor Constantine the Great over Maxentius (for whom the Equites singulares augusti, the emperor's mounted bodyguards had fought) at the Battle of the Milvian Bridge, the guard was abolished and the fort demolished. Substantial remains of the fort lie directly beneath the nave.

en.wikipedia.org