The monumental Arch of Janus (Arco di Giano), the only four-sided (quadrifrons) arch to have survived in Rome, dates back to the 4th century BCE.
Many scholars think that the traditional attribution of the arch to Janus, the Roman god of doors and passages, is not correct. It has been suggested that the arch was, in fact, dedicated to the emperor Constantine the Great (r. 306-37), who was deified after his death, and was known as the Arco Divi Costantini (Arch of the Divine Constantine).
Each pier of the arch has two rows of three niches, and it seems reasonable to assume that some, or all, would once have housed statues.
In the Middle Ages, the Frangipane family transformed the arch into a fortress, and so it survived intact until 1830, when the attic and top part were torn down in the mistaken belief that they did not belong to the original structure.