The Campus Martius (Latin for the “Field of Mars” where Roman heroes walked, Italian Campo Marzio), was a publicly owned area of ancient Rome about 2 km² (600 acres) in extent. In the Middle Ages it was the most populous area of Rome. The IV rione of Rome, Campo Marzio, which covers a smaller section of the original area, bears the same name.
Before the founding of Rome, The Campus Martius was a low-lying plain enclosed on the west by a bend of the Tiber River near Tiber Island, on the east by the Quirinal Hill, and on the southeast by the Capitoline Hill.
According to one legend, the Campus Martius was once a field of wheat owned by Tarquinius Superbus, last King of Rome, but was burnt during the revolution which established the Roman Republic.
In the first centuries after the city’s founding, the area was still outside the Servian Wall. The Campus was used for pasturing horses and sheep, and for military training activity of both the army and of private people who could use the training equipment the army had left. As such, it was dedicated to Mars, the Roman god of war, with an ancient altar and became closely linked to soldiers and the army. Initially, the field was often used by soldiers for purposes of training. Later, it was frequently the focus of Triumphs, the celebrations of successful military campaigns.
Because at the time it was outside the city walls, the Campus Martius was a natural place for audience given to foreign ambassadors who could not enter the city, and foreign cults were housed in temples erected there.
In 221 BC, the Circus Flaminius was built on the southern side of the Campus Martius, near the Tiber. This large track for chariot racing was named after Gaius Flaminius Nepos, who also constructed the Via Flaminia.
Starting in the time of Sulla, building lots were sold or granted to influential Romans, and insulae (apartment blocks) and villas encroached on the common land. It later became the place for comitia centuriata, civic meetings with weapons, and for the city’s militia. Pompey built the first stone theater in Rome in the Campus Martius in 55 BC: this was the first real monument in the area. When the Curia Hostilia burnt down in 52 BC the theater was sometimes used as meeting place for the Senate. It was here that Julius Caesar was murdered in 44 BC. The area was also used as the meeting ground for elections. Julius Caesar planned for the Saepta (enclosures used for elections) to be placed there; they were later completed by his heir Augustus. In 33 BC Octavian dedicated the Porticus Octaviae, built from spoils of the Dalmatian War.
During the Augustan period of the early Roman Empire, the area became officially part of the city: Rome was split up into 14 regions, and Campus Martius was divided into the VII Via Lata on the east and the IX Circus Flaminius nearer to the river.
The Campus Martius also held the Ara Pacis (Altar of Peace), built by the Senate to mark the establishment of peace by Augustus. It was intended to symbolize the successful completion of Augustus’s efforts to stabilize the Empire.
Marcus Vipsanius Agrippa had the original swampy ground made into a pool and baths in a setting of parkland and temples, the Laconicum Sudatorium or Baths of Agrippa, and he built the Porticus Argonautarum and, most notably, the Pantheon, which was later rebuilt by Hadrian as it still stands today. In 19 BC he additionally completed the Aqua Virgo to supply water to these new baths and fountains.
In the non-populated northern area there was the huge Mausoleum of Augustus. Other buildings were made: the Theater of Marcellus, the temple for Isis (from around the time of Caligula), the baths and bridge by Nero.
After the great fire of the 80, Domitianus rebuilt the burnt monuments plus a stadium (eventually to become today’s Piazza Navona) and an Odeion (a small performance hall).
Gradually, the Campus filled with temples and public buildings, circuses, theaters, porticoes, baths, monuments, columns and obelisks. Interestingly, even though the area was originally named for Mars, there was no monument dedicated solely to him in the later Roman period.
Although the region had been left outside the earlier walls, it was finally protected defensively when the Aurelian Walls were built around 270.