The complex grew in five successive campaigns over roughly 160 years. Julius Caesar inaugurated the first new square in 46 BCE, dedicated to his patron goddess Venus Genetrix. Augustus added his own forum in 2 BCE, after some forty years of construction, centred on the Temple of Mars Ultor. Vespasian followed in 75 CE with the Templum Pacis, which housed the Forma Urbis Romae, a vast marble plan of the city. Nerva completed the narrow Forum Transitorium begun by Domitian. Trajan’s Forum, the largest and the last, was inaugurated in 112 CE, with Trajan’s Column erected the following year. Each emperor extended the civic centre of Rome further north from the Republican Forum, in a deliberate sequence of architectural one-upmanship.

Trajan’s Forum was designed by Apollodorus of Damascus, the engineer who had bridged the Danube during the Dacian Wars and funded the work from Dacian spoils. The piazza measured roughly 300 by 185 metres, paved in Carrara marble and closed to the north by the Basilica Ulpia. Behind the basilica rose Trajan’s Column, a 38-metre shaft wrapped in a continuous spiral relief of the Dacian campaigns, flanked by a Latin and a Greek library. To the east, built into the Quirinal hill, the Markets of Trajan formed a multi-level brick complex of more than 150 rooms that buttressed the excavation and served administrative and commercial functions. Visiting Rome in 357 CE, the emperor Constantius II, according to Ammianus Marcellinus, described the forum as a construction unique under the heavens.

The forums lay buried under the Alessandrino quarter, a dense Renaissance and medieval neighbourhood, until the excavation campaign of 1924 to 1932 cleared more than three hectares and displaced 746 families to the city periphery. The avenue laid across the site, first called Via dei Monti and then Via dell’Impero, was opened by Mussolini on 9 April 1932; it took its current name, Via dei Fori Imperiali, after the Second World War. Today the exposed remains form an open archaeological park visible from the street, while the Markets of Trajan house the Museo dei Fori Imperiali, the only ticketed enclosure of the complex.