The Vatican’s history as the seat of the Catholic Church began with the construction of a basilica over St. Peter’s grave in Rome in the 4th century A.D. The area developed into a popular pilgrimage site and commercial district, although it was abandoned following the move of the papal court to France in 1309. After the Church returned in 1377, famous landmarks such the Apostolic Palace, the Sistine Chapel and the new St. Peter’s Basilica were erected within the city limits. Vatican City was established in its current form as a sovereign nation with the signing of the Lateran Pacts in 1929.

The area off the west bank of the Tiber River that comprises the Vatican was once a marshy region known as Ager Vaticanus. During the early years of the Roman Empire, it became an administrative region populated by expensive villas, as well as a circus built in the gardens of Emperor Caligula’s mother. After much of Rome was leveled in a fire in A.D. 64, Emperor Nero executed St. Peter and other Christian scapegoats at the base of Vatican Hill, where they were buried in a necropolis.

Having embraced Christianity with the Edict of Milan in 313, Emperor Constantine I began constructing a basilica over St. Peter’s tomb in 324. St. Peter’s Basilica became a spiritual center for Christian pilgrims, leading to the development of housing for clergymen and the formation of a marketplace that became the thriving commercial district of Borgo.

Following an attack by Saracen pirates that damaged St. Peter’s in 846, Pope Leo IV ordered the construction of a wall to protect the holy basilica and its associated precincts. Completed in 852, the 39-foot-tall wall enclosed what was inaugurated Leonine City, an area covering the current Vatican territory and the Borgo district. The walls were continually expanded and modified until the reign of Pope Urban VIII in the 1640s.

Although the pontiff traditionally lived at the nearby Lateran Palace, Pope Symmachus built a residence adjacent to St. Peter’s in the early 6th century. It was expanded hundreds of years later by both Eugene III and Innocent III, and in 1277 a half-mile-long covered passageway was assembled to link the structure to Castel Sant’Angelo. However, the buildings were all abandoned with the shift of the papal court to Avignon, France, in 1309, and over the next half-century the city fell into disrepair.