Janus kept the gate of Heaven, so he became the god of doors and gates. He was very important, because a house is only as strong as its doors.
His temple in Rome had its doors thrown open in times of War, and closed in times of Peace. They were usually open! The Emperor Augustus closed the doors of the temple, since he brought peace to the Roman Empire. Janus had two faces, one looking forwards and one looking back since a door can let you in, or let you out. The first month of the year is named after him. In January, we look back at the last year and forward to the next.