One entered the Forum of Trajan through the single fornix (passage way) of a triumphal arch. At the center of a vast square, paved with blocks of white marble and flanked by porticoes, behind which opened two hemicycles, was a large bronze equestrian statue of Trajan. At the end of the square, and closing it off, was the Basilica Ulpia, and, behind it, Trajan's Column, flanked by Greek and Latin libraries. Out of view in this picture was the Temple of Divine Trajan, built by Hadrian after his death.
The destruction of the forum began in the fourth century, when relief panels and the Dacian captives were incorporated into the Arch of Constantine. In AD 663, the Byzantine emperor Constans II removed some of the forum bronze statues and ornaments. A serious earthquake in AD 801, which was felt as far away as Germany, probably destroyed much of the forum, and, by the early eleventh century, a small church had utilized the base of the Column of Trajan as one of its walls, the roofline still visible cut into the Latin inscription. In the Middle Ages, much of its sumptuous colored marble was removed and other marble robbed for use in contemporary buildings or sculpture, although a decree of 1162 did declare, upon pain of death, that Trajan's Column was to be preserved, a solicitude that was maintained by subsequent popes but not extended to other parts of the forum, which continued to be exploited, especially in the sixteenth century, when the construction of new churches necessitated increasing excavation. Four giallo antico columns from the porch of the basilica, in fact, were removed to the transept of St. Peter's.
The first systematic excavation of the forum was by the French in 1811-1814, after the occupation of Rome by Napoleon. In 1928-1934, Mussolini approved further exploration, but covered half the forum by the Via dell' Impero, a triumphal avenue inaugurated in 1932.
The site now lies some fifteen feet below street level, as can be seen here, where one almost is at eye level with the capitals of the columns.
Interestingly, "fornication" derives from fornix, as such illicit sexual activity often took place beneath arches or similar vaulted spaces.
This detail is from the Model of Rome.