Before the Empire had begun, the Roman Forum had become too small for the growing needs of the city. Julius Caesar had diminished its area in giving it the present form; in compensation, he built a second forum, called by his name, not as a market but as a centre for business of other kinds. This Forum Julium, which lay to the north, was the first step in a great plan carried on by later emperors for connecting the Roman Forum with the populous quarter of the Campus Martius, or Field of Mars, to the north. Augustus added another forum north of that of Caesar. Vespasian added his great Forum and Temple of Peace, where the spoils of the Temple of Jerusalem were placed, and in A.D. 97 or 98, the Emperor Nerva dedicated a smaller forum, begun by his predecessor Domitian, between those of Augustus and Vespasian, to serve as a passageway northeast to the Esquiline hill. From this use, it is often called the Forum Transitorium.
Little is known of Nerva's forum through written records, but artists have loved it and have drawn it oftener than any other except the Roman Forum itself. The ruins of a considerable part of its buildings stood until the seventeenth century, and the fragments which remain still exhibit rich and delightful decoration.
Aside from foundations and broken bits of columns and carvings, all that is visible today is part of a colonnaded enclosing wall, which once surrounded the forum. Attached to this wall are two Corinthian columns and a fragment of frieze and attic above them. On the attic is a relief of Minerva, Domitian's favourite deity, to whom the forum's temple was dedicated; on the frieze are reliefs showing women busy with the household tasks of which Minerva was patroness. One probably represents Arachne, the mortal who boasted that she was superior to the goddess in the art of weaving and was punished by being transformed into a spider. This beautiful fragment, until recent years enclosed in the walls of comparatively modern buildings, has long been known as the Colonnacce.
Minerva's temple stood at the northern end of the narrow forum. Drawings of the fifteenth and sixteenth century show considerable portions of it still standing; even then, however, the Colonnacce looked much as it does today.
Master Gregory, who described the temple in the twelfth century, wrote of it: 'The temple of Pallas was once an outstanding building, but it was pulled down with great effort by the Christians, and also fell into ruin because of great age. Since it was impossible to demolish it entirely, what remains is now the grain storehouse of the cardinals. Here is a great heap of broken effergies, and here is a headless image of Pallas, armed, standing on the apex of the pediment, a marvel to beholders.
'This image was much venerated among the ancient Romans. They brought Christians before it, and if they refused to bow the knee and worship Pallas, they were tortured to death. It was before this idol that Hippolytus was brought, with his household, and because he scorned it he was torn to pieces by horses.'
This story of a Christian martyrdom echoes the Greek legend of Hippolytus, son of Theseus, killed by Poseidon's horses because he had scorned Aphrodite for the sake of the chaste huntress, Artemis.
The name 'Temple of Pallas' was kept alive through an account in the Acts of the Martyrs. The Mirabilia, however, calls the forum and temple 'the Forum of Nerva with his temple of Divus Nerva', probably because that emperor's name was prominent in the inscription below the pediment. The image that Gregory describes is clearly not that remaining on the Colonnacce, which has its head and is in relief. In 1616 most of the remains of the temple were torn down and used as material for other buildings, especially the great fountain of the Acqua Paolo on the Janiculum, named from Paul V who demolished the ruined temple.
The ground-level about this forum rose in the course of centuries, until today the excavated portions of the Colonnacce lie far below the street. The columns were not uncovered to their full depth or the surrounding houses completely removed until the early 1930's.