Here is the Arch of Janus, a tetrapylon arch (so having  four arches) that is marble and travertine made. It is located near the Church of San Giorgio al Velabro and the curious Arch of the Argentari. The Arch of Janus can be considered like one of the ancient symbols of Rome. 

You can see a series of niches that once were filled with marble statues. This extraordinary monument, the only example of its kind in Rome, was probably the entrance to the area of the Forum Boarium, the ancient meat and livestock market  of Rome, that was built in the city already at the time of the kings. But if the area was indeed archaic, ancient even for the Romans themselves, the Arch of Janus represents, in reality, an architectural monument of the imperial age which will lead, later, at the end of the Western Roman Empire. The Arch of Janus was, in fact, erected in the IV century d.C. at the time of Constantius II, Constantine's son. When the latter died the empire was divided between his three sons, who later fought between them. Constantius II was one of the three, the one that resisted to ambushes and wars who was able, as he did to his father, to be master of the whole Empire. The Arch of Janus was not only an entrance but also an area useful for rainy days and to carry out business in the fresh and, also, a little hidden. Moreover keep in mind that the Arch of Janus is a very ancient monument, so much that, as happened for example at the Colosseum, it was really reused and transformed over the centuries. During the Middle Ages the Frangipane family, one of the most powerful clans of that time, fortified it, closing the arches and turning the Arch of Janus in a real fortress! But who was Janus?

Janus was the two-faced god of Rome, a native deity represented the beginning and the end, the good and the bad, peace and war. In short, all the opposites of our existence. Not surprisingly, the month of January derives from Janus: the beginning of the new year and the end of the next one. It is likely, however, over the four-fronted arch was not dedicated to God (I have never been found neither statues nor guidelines upon him). The Latin name of Ianus, in fact, also meant the entrance door. Exactly the function of this monument, which had also statues of Juno and Ceres, the goddess of the harvest, agriculture, and abundance (a clear reference to the market). Aside from its function, however, there are many doubts that arise when we speak of the Arch of Janus. But the surprises are not over...

First of all, as a result of a complex work of cleaning up and restoration of the monument ended a few years ago, was found an inscription to show that as the arc four-fronted arch was erected in honor of Constantine, the father of Constantius II. But to increase even more the doubt there are the remains of an inscription that was found just near the arch four-fronted arch, and that was re-used over the centuries as the material of reuse, that may well be just the inscription on the attic of the arch. And in that case, by the fragments that we have, the registration may postpone the victory over the usurper Magnenzio by Constantius II. And if this was true, the Arch of Janus may, originally, have been maybe a triumphal arch. Who knows...