To build this monumental complex, extensive excavations were required: workers eliminated a ridge connecting the Quirinal and Capitoline (Campidoglio) Hills.

[4] The arch was flanked by tall walls built from blocks of Peperino tuff clad entirely in marble, which enclosed the Forum on three sides.

The building consisted of three parallel halls separated by annexes and was known as the Athenaeum; it functioned variously as school, a venue for judicial proceedings, and an occasional meeting-place for the Senate.

[9] Constantius II, while visiting Rome in the year 357,[10] was amazed by the huge equestrian statue of Trajan and by the surrounding buildings: But when he [Constantius II] came to the Forum of Trajan, a construction unique under the heavens, as we believe, and admirable even in the unanimous opinion of the gods, he stood fast in amazement, turning his attention to the gigantic complex about him, beggaring description and never again to be imitated by mortal men.

So renouncing all hope of attempting anything of the kind, he said he wanted to imitate only Trajan's horse, set in the middle of the atrium, and with the emperor on its back.

And prince Ormisda ... standing beside him, replied with pleasing wit: "First, emperor, command the construction of a stable like this, so that the horse you wish to have made can find as appropriate a setting as that we have before our eyes.

By the 10th century the Imperial Fora were semi-rural, with a patchwork of houses and farmland crisscrossed by roads occupying the former plaza of Trajan's Forum.