This massive triupmhal arch was constructed in 315AD by the emperor Constantine to commemorate his victory over his rival Maxentius. By this time, the quality of art and architecture in Rome had changed dramatically and such arches were now a rarity. As such, though the arch itself and some carving date from 315, much of the decoration was stolen from earlier triumphal arches that are now long gone, including the lost arches of Trajan, Hadrian and Marcus Aurelius. The arch was constructed before Constantine's much vaunted baptism as a Christian and so still contains pagan imagery.

Access: Freely accessible for viewing at all times.

Though only the base survives, this was once the site of a colossus (after which the Flavian amphitheatre was later named) representing the emperor Nero. This massive marble and gold statue stood 120 ft high (higher than the Colossus of Rhodes as Nero decreed) and originally dominated one of the entrances to the Domus Aurea until Hadrian moved it to its later site. After Nero's damnation, the statue was converted into one of Apollo. Briefly, the statue was changed once again by Commodus to portray himself as Hercules, but was again converted to Apollo after his death. History does not reveal what happened to the Colossus, but it had fallen by the time the Venerable Bede visited Rome.

Access: Freely accessible at all times.

This great gladiatorial training school was originally constructed under Domitian, just after the completion of the Colosseum. The structures currently visible, however, date to its rebuilding under Trajan. In order to facilitate the movement between the Ludus and the Flavian arena, a tunnel connected this structure with the Colosseum. At the centre of the complex was a small arena for gladiatorial practice with a limited number of seats (this was not a site for public performances.) Around the arena were the various other buildings, including the barracks of the gladiators, utility rooms, guard rooms etc. It is not difficult standing beside this ruin to picture the life of a gladiator before his time in the arena. The whole structure is visible in a recessed area to the east of the Colosseum, surrounded by roads and pavement cafes.

Access: Freely visible from above at all times.

The Meta Sudans was a huge monumental fountain in the shape of a cone down which water ran equally. The fountain was constructed some time in the second half of the 1st century AD and lasted beyond the fall of Rome only to be destroyed during the time of Mussolini. Since the change in Rome's traffic system this whole area has become pedestrianised and the base of the fountain brought to light. Nearby are some of the foundations of one wing of Nero's palace that overlooked the lake on which the Colosseum was built.

Access: Freely accessible at all times

The church of San Clemente is a total surprise. At ground level, the only sign of anything pre-Medieval is a small yard or cloister that is surrounded by a portico of re-used Roman columns (photo below). However, the bulk of the remains are considerably below ground level. Unfortunately, photography is forbidden, so I have no images for you. Beneath the church is a massive labyrinthine complex of rooms and tunnels that date back to the 2nd century AD at least. They include a well-preserved mithraeum of the 2nd century, the house of the early Christian martyr Titus Flavius Clemens, and the early 4th century basilica that was dedicated to Pope Clement I. The whole complex shows signs of reuse and alteration throughout history and is truly fascinating. Although not relevant in terms of Roman history, the complex also includes the 10th century rebuild of the church with amazing frescoes.

Access: Open normal church hours. Access to the underground complex requires payment and photography is forbidden. Unsuitable for wheelchairs.