After its period of glory, the Forum was relentlessly attacked for centuries by barbarians, after which it was gradually dismantled by anyone - from popes to paupers - who needed building materials. There is, clearly, a lot missing; but if you bear in mind that for a thousand-plus years this was treated as little more than a quarry, what's really incredible is how much has survived. With a bit of patience (and a lot of imagination), it's possible to reconstruct what was once the heart of the Western world.
During the early years of the Republic an open space with shops and a few temples sufficed, but from the second century BC, ever spreading, ever conquering Rome needed to give an impression of authority and wealth. Out went the food stalls; in came law courts, offices and immense public buildings with grandiose decorations to proclaim the power of Rome. Space soon ran out, and emperors began to build the new Imperial Fora. But the Foro romano remained the symbolic heart of the Empire, and emperors continued to renovate and embellish it until the fourth century AD. After taking the brunt of barbarian violence, the low-lying land of the Forum was gradually buried by centuries of flood sediment which rose unchecked in a city dramatically depopulated by plague, pestilence and war. The ancient heart of caput mundi came to be known as the campo vaccino (cow field). Major excavation didn't begin until the 18th century, when digging for antiquities became the hobby of choice for aristocrats from across Europe.
Our tour of the Forum starts at the entrance closest to the Colosseum, where the cobbled via Sacra - the Sacred Way, which ran through the Forum, past its most important buildings - climbs past the towering columns (on the right) of the Temple of Venus and Roma, the two goddesses who protected the city. You can pick up an audioguide (€4) from the office on the left after the Arch of Titus - worth doing as there's very little identification and a total lack of explanation of the Forum's masonry.
At the top of the rise is the arco di Tito (arch of Titus), built in AD 81 to celebrate the sack of Jerusalem by the Emperor Vespasian and his son Titus ten years earlier. One of the bas reliefs shows Roman soldiers with their plundered prize from the Temple of Herod: the menorah and the sacred silver trumpets. The triumphal procession is shown on the east interior wall, with Titus himself accompanied by a winged Victory driving a four-horse chariot. In the centre of the vault there's a square panel that shows Titus riding to the heavens on the back of an eagle, an allusion to his apotheosis.
After the arch bear right, past the church of Santa Francesca Romana (generally closed to the public, but see Santi Cosma e Damiano) and continue on to the towering brick ruins of the basilica di Massenzio, begun in 306 by Maxentius but completed in 312 by Constantine. Probably the last really magnificent building constructed before Rome began her decline, its marble-clad walls occupied three times the space now covered. The great vaults are considered to have inspired Bramante when he was designing St Peter's.
Retrace your steps to the via Sacra. Passing the brick remains of a medieval porticus you come to a building with a set of bronze doors: this is the so-called temple of Romulus, built in the fourth century and named after the Emperor Maxentius' son, to whom it may or may not have been dedicated. The interior of the temple is under seemingly constant restoration, but is visible from the church of San Giovanni in Laterano.
Also on the right are the great columns of the temple of Antoninus and Faustina honouring Hadrian's successor, Antoninus Pius, and his wife Faustina, who pre-deceased him. In the seventh or eighth century it became part of the church of San Lorenzo in Miranda, which was heavily remodelled in the 17th century. The distance between the level you're standing on and the bottom of the church's (now-redundant) door is a clear example of the difference between the ancient ground level and that of the 17th century when the new façade was added - a millennium of accumulated mud and filth.
Continuing towards the Capitoline, what's left of the giant basilica Aemilia takes up nearly a whole block. Finished in 34 BC by Lucius Aemilius Lepidus Paullus with cash he got from Julius Caesar in exchange for his support, it was once a bustling place for administration, courts and business. A large chunk of a dedicatory inscription can be seen on the via Sacra corner. Beyond is the Curia. The home of the Senate, it was begun in 45 BC by Julius Caesar and finished in 29 BC by Augustus. (Still under construction on the Ides of March 44 BC, it wasn't here that Julius Caesar was assassinated but at the Curia of Pompey) It was rebuilt after the fire that heavily damaged the Forum in AD 283; in the seventh century AD it became the church of St Hadrian, almost all trace of which was removed during Mussolini's restoration of 1935-8, although some fresco fragments still cling on inside. The floor is a fabulous example of fourth-century opus sectile. The doors aren't the originals, though - they found their way to in 1660. Opposite, directly in front of the arch of Septimius Severus, is a fenced-off, irregular patch of greyish limestone: this is the lapis niger (black stone) that was believed to mark the tomb of Romulus.
The arch of Septimius Severus was built in AD 203 to celebrate a victory in Parthia (modern-day Iran). The reliefs of military exploits are now blurred, but some of those at the base of the columns are better preserved, having been buried until the 19th century. These show Roman soldiers (with no head-gear and wearing shoes) leading away their Parthian prisoners (with downcast faces and floppy Smurf hats).
Just to the left after you pass through the arch is a circular brick structure believed to be the umbilicus urbis (the navel of the city). Next to it, the curved white steps are all that remains of the rostra of Julius Caesar, a platform for the declamation of speeches. Towards the Capitoline hill, the eight massive columns of the temple of Saturn bear an inscription that says it was rebuilt after a fire (probably around AD 360). The cult of Saturn was an ancient one: the first temple to him was built here around 497 BC. The feast dedicated to Saturn, the Saturnalia, was celebrated for three days beginning on December 17 and turned the social order on its head, with slaves and servants being served by their masters. With your back to the temple of Saturn, the solitary column of Phocas is clearly visible. The last monument erected in the Forum before the area became a quarry for Christian structures, it was dedicated to the murderous emperor of Byzantium, Phocas. The inscription on the pedestal records a gold statue erected on top in 608 by Smaragdus, Exarch of Ravenna (although the column itself is recycled, probably dating from the second century).
On the far side of the via Sacra stand the foundations of the basilica Julia, built by Julius Caesar in 55 BC and once a major - and by all accounts very noisy - law court. Following the basilica to the right, you come across an opening to the Cloaca Maxima. At the end of this path is a fenced-off area around the church of Santa Maria Antiqua, where beautiful seventh- and eighth-century frescoes were under restoration as this guide went to press. Rising beside the basilica Julia are three elegant columns that formed part of the temple of Castor, one of the twin sons of Jupiter who, according to legend, appeared to Roman troops in 499 BC, helping the Republic to victory over the Latins.
Back towards the centre of the Forum is the temple of Divus Julius. A nondescript mass of concrete masonry beneath a low-pitched green roof is all that remains of the temple dedicated to the memory of the deified Julius by Augustus in 29 BC. It's popularly held to be the place of Caesar's cremation (though in fact this took place at the other end of the Forum, across the via Sacra from the temple of Antoninus and Faustina), and flowers are still left here on March 15 (the Ides) each year.
Between the temples of Castor and Divus Julius are the scant remains of the arch of Augustus, believed to be the arch built in 29 BC to commemorate his victories, including that at Actium over Antony and Cleopatra in 31 BC. Just beyond, three small columns arranged in a curve mark the round temple of Vesta where the vestal virgins tended the sacred fire. Within its garden, the rectangular house of the Vestal Virgins was where they lived chastely: if not they could expect to be buried alive in the 'field of wickedness' (campus scleratus) by the Quirinale.