Campus Martius is the Roman district that boasts some of the most famous monuments in the world.
During the age of ancient Rome, it was an extensive area outside the official city boundaries (the Pomerium), occupying a large, flat region at a bend in the Tiber to the north of the Quirinale and the Campidoglio.
Tarquinius Superbus (Tarquin the Proud), Rome's last king, turned the area into an enormous wheat field. According to legend, during the revolt that ultimately overthrew the king, sheaves of grain were thrown into the Tiber, creating Tiber Island (in actuality, this is only one of many anecdotes about the island's origins).
The name Campus Martius comes from the age of Roman Kings. It was dedicated to Mars, the god of war, and an altar to the god was erected at the site. In fact, the historian Livy relates that Tarquin's field dedicated to Mars has been used for military practice and exercises since Rome's founding. In the age of the Republic, the army continued to gather here.
Because it was outside the Pomerium the sacred boundary of the city it came to be considered as a kind of neutral ground. This is where foreign ambassadors were received. (Largo Argentina, part of the Campus Martius, was so named because it was next to the residence of the ambassador of Argentoratum, now Strasbourg). Temples dedicated to oriental divinities were also built here.
In the southernmost part of this area, at the base of the Capitoline Hill, the remains of the Theatre of Marcellus and the Portico of Ottavia are visible. Campus Martius was not inhabited until the Imperial age, when it joined the part of the city the Emperor Augustus had named "Circus Flaminius." The area was accessed by the Via Flaminia, passed through the Porta del Popolo, and ended in what is today the Via del Corso.
The Theatre of Pompey was the first significant work built in the area. It was also the first Roman theatre built in masonry (previously they were temporary wooden structures). While the theatre no longer exists, its remains and original outline can still be seen in the palazzos on the Piazza of the Theatre of Pompey, near the Campo dei Fiori.
During the Augustan Age, Agrippa created public baths at Campus Martius bearing his name, as well as the Pantheon. The first permanent Roman amphitheatre was also built in this area; called the Theatre of Balbo, it is partly visible in the museum of the Crypta Balbi between the Largo Argentina and the Piazza Venezia. During this period, a large sundial was constructed in the Campus Martius. Using an obelisk for a gnomon, it was sited near the Ara Pacis and constructed so that on the emperor's birthday, the obelisk's shadow would reach the Ara Pacis, the monument that embodied the Augustan period of peace and stability.
After the great fire of Rome in 80 A.D., Emperor Domitian rebuilt all the monuments and created one of the most impressive works of Rome: the Circus of Domitian, on top of which the Piazza Navona was later to emerge. Emperor Hadrian made improvements to the Pantheon and later dedicated a temple in this area. And finally, two columns were erected in the Campus Martius. One of the columns, dedicated to Antonino and Antonina Pio by Marcus Aurelius, can still be found at its original position in Piazza Colonna, in front of the Palazzo Chigi. Both columns were inspired by the celebrated Trajan's Column.
Because it was close to both the river and St. Peter's Basilica, Campus Martius became a pilgrimage centre during the Middle Ages and one of the most populated areas in the city. It was also intersected by the processional road the Pope used when he was carried in great regalia from St. Peter's to the Basilica of St. John Lateran, where he still resides.
During the era of Papal Rome, this area underwent several changes, especially to the roadways, including the creation of Via del Corso and Ponte Sisto, roads which today run along the edge of the Tiber, and Via del Babuini, one of the three streets forming the trident that begins in the Piazza del Popolo. In 1570 the Virgin Aqueduct was restored; originally commissioned by Augustus, it still supplies water to the Old City today. The restoration allowed for numerous fountains to be built, including the Trevi Fountain. There was no shortage of palazzos or, naturally, churches built as well.
To prevent flooding, the Tiber was enclosed in 1870 by high stone embankments and lined by the Lungotevere (embankments), one of which is named the Lungotevere Martius.
In 1900 the ancient Campus Martius like other areas of the city was demolished to create new roads. A large travertine piazza was created around the Mausoleum of Augustus, work which required the destruction of about 120 ancient houses.