It was probably a honorary arch dedicated to emperor Constantine and built in the 4th century using the fragments from the other constructions. In the Middle Ages it was turned into a base of the fortified fortress by the Frangipani family. But in 1830 it was returned its original look, with the exception of the attic, which was destroyed by mistake, taken for a part of the fortress. Every of the four sides of the arch had niches with 12 little statues of different divinities inside.