Rome started small and the forum did too, but it always was crowded. And at the height of ancient Rome's population (1.5 million or so in the immediate urban area), the forum teamed with persons of all stations and classes and from dawn to well into the night. If you weren't important enough to go on your own business, you might be dragged along to swell the entourage of your boss -- it was important for bosses to impress other bosses by moving about in this center of activity with huge flocks of underlings. Even underlings had underlings, and they all went to the Forum. Rich kids (who had underlings of their own) went to the Forum to hang out -- it was like the shopping malls in rich American suburbs, but on a much grander scale. The only places more popular were the Colosseum, right at the eastern end of the Forum, and the Circus Maximus, around the corner of the Palatine, when games were in progress. And the games were held on non-business days in the Forum.
There were really several forums in Rome. There was the Republican forum, usually designated the Forum Romanum and the topic of this piece. When the Forum Romanum got to crowded to continue to host the vegetable and meat markets, they were moved to the forum holitarium and forum boarium, which were closer to the River but still connected to the Forum Romanum by the Velabrum valley. And, as space got tighter and crowds bigger, the various "Imperial Forums" were built just north of the Forum Romanum: Julius Caesar's forum; the forum of Augustus; the forum of Vespasian (forum of Peace); the forum and market of Trajan; and the narrow forum of Nerva, sometimes called the forum transitorium because it connected the seedy Subbura neighborhood to the north with the Forum Romanum.
The marshy area that became the Forum Romanum lay in the depression that separated the Quirinale and Opian hills to the north from the Palatine to the south. The eastern end of the forum was crossed by the low Velia ridge that connected the Opian and the Palatine hills, and the western end of the forum butted up against the base of the high Capitoline hill. The Velabrum valley between the Capitoline and Palatine hills connected the Forum to the Tiber River, and it was through this valley that the waters of the Tiber entered the Forum whenever the river rose.
The river rose often. In the earliest times, the Capitoline Hill was sometimes completely surrounded by the waters of the swamp and the Forum could easily be completely submerged whenever the Tiber crested. That was all it took to flood the marsh, and the water stayed long. The Palatine hill was also almost completely surrounded by a very wet marsh. South of the Palatine, the area where the Circus Maximus was later built was mostly submerged, and there was marsh to the east of the Palatine and a small lake in a depression where the Colosseum now stands. The only dry connection between the Palatine and nearest high ground, the Opian Hill, was that Velia ridge.
People lived on the hilltops, and, when it was dry enough to do so, they traded in the lower, neutral territories, especially in the slopes of the Velia ridge facing westward into what eventually became the Forum Romanum. A few early temples, including the all-important Temple of Vesta housing the sacred fire, were built on the Velia slope, and that end of the forum valley acquired the religious character that it would always have.
To understand how the central area of the forum got its commercial role, we have to dip into Roman history/mythology. The beginning of Rome found Romulus and his band of outlaws (he actively recruited them) on the Palatine Hill. They had already downed Remus, whose band was on the Aventine Hill south of the Circus Maximus valley. Although the Roman "founding myths" say that Romulus and Remus were descended from the Trojan Aeneas through kings of Alba Longa, the names Romulus and Remus are actually Etruscan. Many experts say that Romulus and Remus were simply Etruscan outlaws or outcasts, who may or may not have been related -- descriptions of them as twins, in other words, might just have meant that they were similar and equally matched, and the founding myths were embellished later to give an appropriate heritage to Romans and especially to the Julian line that had paid Vergil to write up the Aeneas legend as an epic.
There were Sabine groups on the Quirinale and Opian Hills, and the opposite, western shore of the Tiber River was Etruscan as was most of the territory north of Rome. Tiber Island, on which the earliest evidence of settlement in the area was found, changed hands often. Latin tribes -- cousins of the Sabines -- filled most of the area just south of Rome.
The story of Romulus getting the Sabine men drunk and stealing and raping their virgin daughters is well known, as is the sequel: the girls just loved being so mistreated and interceded to stop the war between their parents and their new "husbands", the Roman outlaws. Once neighborhood peace was assured, regular trade between the Sabines and Romans and, eventually, a summer market were established in the forum valley. And within a few generations Etruscan kings had taken the Roman monarchy and brought their formidable engineering skills to bear to drain the valley and make the forum an all-season market. First, open sewers (cloacae in Latin) were dug, and, during the reign of the last Etruscan monarch, Tarquinius Superbus (Tarquin the Proud), the great covered sewer, the cloaca maxima, was installed. The river still rose often, flooding the forum and the Campus Martius, north of the Capitoline Hill, but the cloaca maxima quickly drained the water from the forum back into the Tiber. Over the centuries, dikes were built along the River to limit flooding, but the cloaca maxima was what made the Forum useable.
Meanwhile, the Capitoline Hill had achieved a dual function. The lower of its twin peaks became the templum or dedicated ground for the Temple of Jupiter. The higher peak was the Arx from which the Augurs, priests whose job it was to interpret the flight of birds, could announce their findings. The only road up to the Capitoline was the Clivus Capitolinus, meaning Capitoline ramp, up from the Forum. At the base of that ramp, in the area near where the Arch of Septimus Severus was later built, the Romans gradually established the buildings and structures associated with their civil government. There was a curia or Senate chamber (on a slightly different axis than the one we see now that Julius Caesar started) and in front of the curia was the circular comitia platform on which the comitia tibuta -- a legislative assembly of the people, organized on a tribal basis -- met. The edge of the comitia platform facing the forum was the tribunal, so named because the tribunes of the people spoke there. The side of the tribunal facing out into the Forum was eventually decorated with the bronze ramming beaks (rostra) of captured enemy warships, and speaking to the mob from the rostrum became the habit of politicians and demagogues. Behind the Clivus Capitolinus, along the side of the Capitoline hill facing the Forum, the Romans built their Tabularium where the "Twelve Tables" of Roman law and other civic and legal documents were kept.
And so, there was the Forum Romanum with three sections: the civil government on the western end, the food market in the center, and the religious area centered on the cult of the Vestals on the eastern end. This tripartite plan had appeared haphazardly, so there was little resistance to its gradual dissolution. Over the 1000 plus years of ancient Rome's power -- from 723 BC, the mythical year of Rome's founding, through 330 AD, when Constantine decamped with his government for Constantinople -- the forum became a hodgepodge of interspersed civic and religious structures. The food stalls and trinket shops were soon expelled to sites closer to the riverbank in the forum boarium and the forum holitarium. New temples were built wherever there was space in the Forum. Vast basilicas were built along the sides, in which government officials and private individuals could conduct legal and civil business and law.
Already by the end of the Republic, the Forum Romanum had become so crowded that Julius Caesar (and succeeding Emperors) could justify massive expenditures for northward expansions that became the Imperial Forums, but, meanwhile, any open spaces in the forum were targets for installations of temples, dedicatory columns, and statues. The forum glittered with gold and shiny new bronze and flashed with color -- remember that many statues were painted, and all of the frescoes and paintings were bright and new.
After Constantine took the government to the eastern Mediterranean the Forum, like the rest of Rome, went into rapid decline. Within a few hundred years rubble, trash, and flood-born soil had raised the surface level in the forum area by several meters, vegetables were grown and sheep and cattle grazed where the great of Rome had once strode. The tops of ruined ancient monuments jutted up and provided support and foundations for much meaner structures as well as inspiration for centuries of artists.
Even when Rome was gradually repopulated beginning in the 14th century, there was no interest in the Forum ruins except as quarries for pre-made pillars, and for stone and lime for new construction. Look for the deep grooves near the tops of the columns on the porch of the Temple of Antoninus and Faustina (http://wings.buffalo.edu/AandL/Maecenas/rome/t_antoninus/ac740825.html): they are from cables Renaissance looters used while trying to jerk the pillars out of the ground for reuse elsewhere. Michelangelo carved a temple step for the base he put under the ancient statue of Marcus Aurelius in the Campidoglio. Stone from the top levels of the Forum is all over Rome, and only the depth of the debris cover kept the rest from disappearing.
Nineteenth and 20th century archeology, continuing into the 21st, has revealed what we see today. We can only hope that the situation in the area never deteriorates again to a level where the now exposed ruins are again seen, not as valued monuments to the past, but as raw construction materials.
There is obviously no room here to discuss the various projects that filled the forums to bursting, but there are some very good Internet sites that can get you started. Here are the links:
The Roman Forum Through the Ages -- at the bottom of a short initial page are links to six detailed pages with internal links to pages about many of the individual Forum structures: http://www2.trincoll.edu/~mzimmerm/zimmerman/forum.html
An interactive picture looking east from the Tabularium showing what the forum looked like in late Roman times, with links to pictures and captions describing important Forum structures: http://www.vroma.org/~forum/forum.html
An interactive line drawing of the forum looking down and north from the edge of the Palatine hill, with links to line drawings of individual Forum structures, captions and photos of what the structures look like today: http://library.thinkquest.org/11402/uitgelicht.html
Roman Architectures page from AncientSites "Encyclopedia Romana" Internet site, with links to the Forum Romanum and Imperial Fora sections (among others): http://itsa.ucsf.edu/~snlrc/encyclopaedia_romana/romapage.html
FORVM ROMANVM -- a good big Internet site on the Forum: http://library.thinkquest.org/11402/
The Republican period Forum -- short history: http://www.humnet.ucla.edu/rome-reborn/Library/LTUR/LTromforRep.html
The Imperial period Forum -- short history: http://www.humnet.ucla.edu/rome-reborn/Library/LTUR/LTforRomImp.html
Pictures from the "Forum Group" of Virtual Roma: http://www.vroma.org/images/forum_project_images/
Pictures from the Maecenas site, many of which are of forum structures: http://wings.buffalo.edu/AandL/Maecenas/general_contents.html#Italy - Rome