The Ager Vaticanus was chosen by Caligula (37-41 AD ) for his circus, which was enlarged by Nero (54 68). The site of the Circus of Nero, just south of the Basilica of St. Peter's, was identified during excavations in the 20th century. In the adjoining gardens many Christians were martyred under Nero in 65 AD, including St. Peter who was buried in a pagan cemetery nearby. Over his grave the first church of St. Peter's was built (ca. 90 AD) to commemorate his martyrdom. Also within the Ager Vaticanus, Hadrian built his mausoleum (now Castel Sant'Angelo) in 135 AD .
Some sources say that the Borgo was founded as a separate residential community in the 6th century by Totila, King of the Goths, but archeological evidence does not agree. Inscriptions found in temples of Cybele and Mithras suggest that Roman pagan communities preexisted and that paganism retained its hold in the area until the late 4th century. Meanwhile, Christians were moving into the area: churches, chapels and convents were built round the first church of St. Peter, and the district, which attracted Saxon, Frank and Lombard pilgrims, came to be called the Borgo (borough), a name of Germanic origin from borgus meaning a small fortified settlement. Most authorities agree that the name originally was applied to the Borgus Saxonum founded by King Ine of Wessex, who abdicated his throne and came to live in Rome in 726 AD. That borgus stood at the site of the present Arciospedale di Santo Spirito in Sassia on the street now named Borgo S. Spirito. Other such communities followed. (The ospedaletto next door to S. Spirito [built by Sixtus IV from 1473-78] has a museum of the history of medicine starting with ancient Roman times and the Palazzo del Commendatore, the historic residence of the hospital director, further on [built 1567-71], has a corresponding medical history library, the Biblioteca Lancisiana.)
In 850 Leo IV (847-55) built 12 meter high walls around the Borgo, fortified with circular towers, to protect the neighborhood from the incursions of the Saracens: Since then, the area has also been known as the Leonine City (Citta Leonina). Remnants of Leo's wall survive to the west of St. Peter's. The Leonine City became the papal citadel: within its walls John VIII was besieged in 878 by the Duke of Spoleto; in 896 Arnulph of Carinthia attacked it and Formosus crowned him emperor. Gregory VII took refuge in the Castel Sant'Angelo from the Emperor Henry, and was rescued by Robert Guiscard in 1084. After the coronation in 1167 of Barbarossa in St Peter's, the Romans besieged the Leonine City (and attacked it again 12 years later).
During the "Babylonian captivity" (1309-78) the Borgo fell into ruin, but when the popes returned from Avignon to Rome they chose the Vatican as their residence in place of the Lateran. In the 15th century Eugenius IV and Sixtus IV, and early in the 16th century Julius II and Leo X were active in developing and embellishing the Borgo as well as the Vatican. The original area of the Borgo was enlarged to the north of Borgo Angelico. However, after the sack of Rome in 1527 the Borgo became one of the poorest and least populated districts of Rome, and in 1586 Sixtus V relinquished the papal claim to this area, so that it was united to the city of Rome.
Five (originally seven) streets in the Leonine City have the prefix Borgo. Borgo Sant'Angelo and Borgo Santo Spirito run respectively north and south of Via della Conciliazione. Via della Concilazione was built between (1936 to 1949) to celebrate the reconciliation of the Papacy and the Italian state achieved by the Lateran Treaty of 1929. In the construction of that street, the central Borghi, Nuovo and Vecchio, were destroyed along with the narrow strip of buildings between them, the spina, so named because of its resemblance to the spina of classical circuses. The remaining Borgo streets, the Borghi Angelico, Vittorio, and Pio survive north of Borgo Sant'Angelo, between the Castel Sant'Angelo and the Vatican.
The passetto, a raised fortified corridor emerges from the palace behind the northern wing of the great elliptical colonnade around St. Peter's Square and runs along Via die Corridor and Borgo Sant'Angelo to Castel Sant'Angelo. Sources disagree on when and by whom the passetto was constructed: either by Pope Nicholas II in 1277 or by the anti-Pope John XXIII (1410-15). Pope Alexander VI Borgia repaired the passetto late in the 15th Century. On some maps it is shown as the Corridore di Alessandro VI. In 1527, Pope Clement VII used this escape route to Castel Sant'Angelo during the sack of Rome.
The famous Ponte Sant'Angelo (pedestrians only), the ancient Pons Aelius or Pons Adrianus, was built by Emperor Hadrian (Publius Aelius Hadrianus) in 134 as a fitting approach from the Campus Martius to his mausoleum, known since the Middle Ages as the Castel Sant'Angelo. Although the Roman bridge was decorated with statues, it was transformed by Gian Lorenzo Bernini when he designed the ten statues on the balustrade of angels holding the symbols of the Passion. These were executed in 1688 by his pupils, including Ercole Ferrata, Pietro Paolo Naldini, Cosimo Fancelli, and Antonio Raggi; two of the angels are copies of the originals which were removed to the church of Sant'Andrea delle Fratte. At the end towards the castle, the statues of St Peter and Paul, by the school of Lorenzetto and Paolo Taccone (1464), were set up by Clement VII in 1534. The three central arches are part of the original structure; the end arches were restored and enlarged in 1892-94 during the construction of the lungotevere embankments.
To the south of Ponte Sant'Angelo is Ponte Vittorio Emanuele (1911), decorated with monumental sculptures in travertine, and bronze victories.
There are only minimal Internet links on the Borgo:
Rione Borgo (map): http://www.mirabilia2000.com/rioni/r14ing.htm
Raphael's fresco (Vatican Stanzae) -- Fire in the Borgo: http://www.hellasonline.gr/cjackson/raphael/raphae45.jpg
Hadrian's Mausoleum/Castel Sant'Angelo (images): http://www.siba.fi/~kkoskim/rooma/pages/MHADRIAN.HTM
S. Pietro/St. Peters (Images): http://www.siba.fi/~kkoskim/rooma/pages/SPIETRO.HTM
Both St. Peters and Castel Sant'Angelo are well represented on the Internet -- use your search engine -- but make sure you type the limiter +rome after St. Peter's or you will access churches all over the world.