Arch of Janus

The Arch of Janus is the only quadrifrons triumphal arch preserved in Rome. It was set up at a crossroads at the northeastern limit of the Forum Boarium, close to the Velabrum, over the Cloaca Maxima drain that went from the Forum to the River Tiber.

It was built in the early 4th century CE, using spolia, i.e. material from earlier buildings, including bricks, together with pottery shards, and was covered with white marble, also from earlier buildings. The exact significance of the arch is poorly understood: it is thought to have been a boundary marker rather than a triumphal arch. An alternative view is that it was built to provide shelter for the traders at the Forum Boarium cattle market. Some researchers believe it was dedicated to Constantine I or Constantius II and was known as the Arch of the deified Constantine. It is 16 meters high and 12 meters wide.

Wikipedia