The temple of Janus was built by Numa Pompilius (716-673 BC). The doors were opened to indicate that Rome was at war and closed during times of peace. Since the time of Numa and before the time of Nero, the doors were said to have been closed only in 235 BC, after the first Punic war; and three times during the reign of Augustus.
Following the favourable end to a war with Parthia in 63 thanks to the efforts of the general Gnaeus Domitius Corbulo, and the general establishment of peace across Rome’s borders by 65, Nero famously closed the doors to great fanfare in AD 66 as a sign that all war was at an end.
