When was the Roman Forum built?
Building activity in the Roman Forum got underway in the 6th century BC, once this former swampland had been drained by the Cloaca Maxima or 'Great Drain.'
There's a big difference between the Forum of the 6th century BC and the site you see today.
Today's Roman Forum is a veritable mish-mash of historical layers. Its oldest monument, the Lapis Niger (Black Stone) contains an inscription from the 6th century BC: the earliest Latin inscription known to date. Its most recent monument, the Column of Phocas, was erected in 608 AD.
Meaning the chronological difference between the earliest- and latest- known monuments spans some 1,200 years.
What was the Roman Forum used for?
At first, the Roman Forum functioned as an open-air market. Soon it became the site of the Comitium, a public gathering space, where the Romans held tribunals, met for political assemblies, and where the most eminent citizens delivered speeches.
By the time of Julius Caesar (the first half of the 1st century BC), the Roman Forum had become cluttered with temples, basilicas, podiums, and meeting spaces. This lack of space necessitated the construction of what we call the 'imperial fora' (built by Roman emperors during the Roman Empire).
Caesar's Forum was the first, soon to be followed by the fora of Augustus, Vespasian, Domitian, Nerva and Trajan just to name a few. The Roman Forum was still populated with monuments from time to time, the Arch of Septimius Severus being a relatively late example. But as the city of Rome grew, so did the focus of its attention shift away from its nucleus.
What did the Roman Forum look like?
A good question, but difficult as the Forum went through so many stages.
As already mentioned, the Roman Forum started off as swampland, an area of uninhabitable terrain between the Palatine and Capitoline hills. Being low-lying, it was susceptible to flooding, though the Etruscans resolved this problem around 600 BC when they built the Cloaca Maxima or 'Great Drain.'
The Roman Forum in the 4th century AD