On a recent journey in the historic center I saw students from Harvard University sketching the "Arch of Janus." This was the center of an area of ancient Rome called the Velabro that linked the wholesale markets with the Roman Forum. In antiquity it was a bustling area with boats unloading cargo and merchants and customers haggling for the best prices. Today is it a quiet place, with the Basilica di San Giorgio in the background.
The "Arch of Janus" is not a regular arch, but a massive travertine cube that was a four-way covered passage, built by Constantine in the 4th century on top of the Cloaca Maxima. It was not dedicated to the Roman god Janus, but probably received the name because the passage could be walked through in two different directions. (The god Janus faced two different directions.)
In the Middle Ages the Frangipane family transformed the building into a fortress. In 1830 an attic and top of the monument were removed because archeologists at the time incorrectly thought that they were not part of the original structure.