Rome rapidly expanded beyond its city limits, conquering the rest of Italy and the Mediterranean littoral, but it deliberately kept an area under its control but beyond its limits -- until the Empire. This area was used for activities deemed unsuited for the city center; however, eventually, the densely populated area that was the historical and administrative heart of the growing empire needed the space that was the Campus Martius or Field of Mars.
Also see The Significance of the Campus Martius.
1. The Campus Martius
PD Courtesy Ancient Rome: Images and Pictures
The Campus Martius was a field and pasture land consecrated to the main Roman war god Mars. Activities unfit for the city center were staged in this adjacent area, but by the time of the Roman Empire, the Campus Martius was needed to house important Roman structures.
2. The Theater of Pompey
CC Flickr User Bert Kaufmann
The complex of the Theater of Pompey was built in the Campus Martius at a time when permanent theaters weren't allowed in Rome proper. Since it was beyond the limits, it provided some security for the conspirators on the Ides of March.
3. Ara Pacis
CC Flickr User Alun Salt.
The Altar of Peace of Augustus or Ara Pacis Augustae commemorated the first emperor's establishment of peace in 13 B.C. It was dedicated in January 30 B.C., rediscovered in 1568, but not identified until 1869 when archaeologist Friedrich von Duhn identified the monument.
4. Pantheon
CC Flickr User Eustaquio Santimano
Augustus' friend Agrippa built the original Pantheon as part of a reorganization of the Campus Martius from 27 to 25 B.C. Although it had to be rebuilt, it has continued to be in use ever since. It was a temple for all the gods. Agrippa's structure appears to have been rectangular.
