It is an honorary arch, an imposing marble-covered monument located at the foot of Via del Velabro descent. The so-called Arch of Janus is a four-sided arch built by Constantinus II in the 4th century AD.
The name of the arch derives from the Latin term “ianus”, the gate, the passage, corresponding to the Arcus Costantini, erected at the center of an important crossing on the edge of the Foro Boario near the ancient piers.