He has to be vigilant because in ancient Rome he was the god who was the guardian of doors and gates.
In his temple in the Roman Forum were two doors. These were kept shut in times of peace to secure the god so that he could guard the city. But in times of war they were opened to allow Janus to go to help the Roman soldiers in battle.
The month of January was sacred to Janus, which is how it gets its present name.
Janus is a relief sculpture
approx. 10" high and 10" wide (25cm x 25cm).