Janus is one of the oldest Roman gods. He was represented
as having two faces: one look forwards, and the other – backwards. Janus
helped the Romans to defend their city, that is why in time of war the
door of the Temple of Janus was always kept open, so that the god could
come to the aid of Romans. The door was only closed if the Roman Empire
was at peace.
See: Peter Paul Rubens Temple
of Janus.