Overview
The Temple of Janus stood in the Roman Forum near the Basilica Aemilia, along the Argiletum. It was a small temple with a statue of Janus, the two-faced god of boundaries and beginnings inside. Its doors were known as the "Gates of Janus", which were closed in times of peace and opened in times of war. There are many theories about its original purpose; some say that it was a bridge over the Velabrum, and some say it functioned as a gate to the Capitoline.
Origins
According to Livy 1.19 the second king of Rome, Numa Pompilius, decided to distract the early, warlike Romans from their violent ways by instilling in them awe and reverence. His projects included promoting religion, certain priesthoods, and the building of temples as a distraction with the beneficial effect of imbuing spirituality. The Janus in the Roman Forum, although not a temple was claimed to be Numa's most famous architectural project.
In the early stages of the city of Rome, the inhabitants lived in separate walled villas. The Janus was the gate from the Forum to the rest of the Rome as originally the Forum was the courtyard of the king's villa.
From Wikipedia (CC BY-SA 4.0).