Keystone on the Arch of Janus in Rome, July 2012

The Arch of Janus is the only quadrifrons triumphal arch preserved in Rome, across a crossroads in the Velabrum-Forum Boarium. It was built in the early 4th century of spolia, possibly in honour of Constantine I or Constantius II. Its current name is probably from the Renaissance or later and is not ancient. The name is derived from its four-fronted, four-arched structure.

In the Middle Ages, the Frangipane family transformed the building into a fortress and so it survived intact up until 1830. Then the attic and top were torn down because they were erroneously believed not to belong to the original structure. Fragments of the dedicatory inscription are still preserved inside the nearby church of San Giorgio al Velabro.

Text from: <a href="http://en.wikipedia.org/wiki/Arch