The Campus Martius itself was a low-lying plain next to the River Tiber. According to one legend, it was once a field of wheat owned by Tarquin, last King of Rome, but was burnt during the revolution which established the Roman Republic. It later became a point of assembly for civic meetings, and of the city's militia.
Dedicated to the god of warfare, the Campus Martius was 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 a successful military campaigns. There was an altar to Mars established on the Campus Martius.
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 symbolise the successful completion of Augustus's efforts to stabilize the Empire.
Gradually, as the city of Rome expanded, more and more buildings were constructed on the Campus Martius. Prominent structures included the Mausoleum of Augustus, the Theatre of Marcellus, the Theatre of Pompey, and the Porticus of Octavia. The area was known for its monuments and temples.