On this day in ancient Rome, in 260 B.C. a temple was built to Janus by C. Duilius after a naval victory over the Carthaginians in the First Punic War. The dedication date was changed during the Imperial period to October 18. Just as the date changed, so did the image of Janus when, somewhat later, under Domitian, the two-faced god was depicted having four faces looking at four Roman forums.
The most commonly mentioned temple of Janus is not this one, which was located in the Vegetable Forum (Forum Holitorium), but the Janus (Geminus) -- technically, shrine -- on the Argiletum. Ronald Syme describes this shrine as
Not a temple, and barely a shrine, but originally a passage: two arched gates with double doors, joined by lateral walls to form a rectangle.[See Forum map showing the Forum Holitorium, Forum Boarium, Argiletum, the Janus (Geminus) shrine, and more.] It was that shrine that had the famous war gateway, with doors that closed in times of peace.
"Problems about Janus," by Ronald Syme. The American Journal of Philology, Vol. 100, No. 1, Tekmhpion. A Special Issue in Honor of James Henry Oliver (Spring, 1979), pp. 188-212
Usually, Janus is shown with two faces, both bearded, as in this illustration. Sometimes one is bearded and one clean-shaven, and sometimes there are four faces.
Read more on Janus.