"But when he came to the Forum of Trajan, a creation which in my view has no like under the cope of heaven, and which even the gods themselves must agree to admire, he stood transfixed with astonishment, surveying the giant fabric around him; its grandeur defies description and can never again be approached by mortal men."
The soldier and historian Ammianus Marcellinus (c.330 - c.391) is referring, in his book Res Gestae, to a visit to Rome in 357 by the emperor Constantius II (r. 337-361), the son of Constantine the Great.
Marcus Ulpius Traianus reigned as emperor from 98 to 117 and the forum that bears his name was constructed under the direction of the Greek architect and engineer Apollodorus of Damascus (died 130).
The Forum, which was built between 107 and 113 to celebrate Trajan's military victories, was the last and most magnificent of the Imperial Fora. It comprised a large colonnaded space (with a bronze equestrian statue of Trajan at its centre), the Basilica Ulpia, and a 100-foot-high column flanked by two libraries.
The purpose of the basilica, the largest of its kind to date, was to provide covered space for the administration of justice. With five aisles and a large semi-circular space at either end, it measured 176 by 59 metres. The central space was 88 by 25 metres and the height of the ceiling is thought to have been in the region of 25 metres. The ground-floor colonnades were made up of Egyptian granite columns (11 m) with an upper colonnade (surrounding the central space) comprising columns of Carystian green marble (9 m). On the side facing the Forum square the basilica was approached by a flight of five steps of giallo antico marble.
All that remains now are a few reconstructed columns, some patches of marble pavement and an assortment of architectural fragments.