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Before the birth of Rome, The Campus Martius itself was a low-lying plain limited by a bend of the River Tiber and by the Quirinal and the Capitoline hills.
As long as the Aurelian Walls were not built, since it was outside the 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.
The Campus Martius contained the main part of Rome until the new developments increased the size of the capital of a reunited Italy after 1870.