Arch of Janus

The only surviving quadrifrons (four-faced) triumphal arch in Rome, standing 16 m high on a 12 m square base in the Forum Boarium. Built in the early 4th century AD using spolia from earlier buildings, it may honor Constantine I or Constantius II. Each of its four passageways measures 10.6 m high, and the piers contain 48 niches across two tiers. The Frangipane family converted it into a fortress tower in the Middle Ages.