Of course, when one looks at St. Peter's or St. John Lateran, it can be easy to think of them as 'modern' structures. In the case of St. Peter's this is true for the reason that the ancient basilica regrettably fell into ruin and was entirely replaced with a new, Renaissance era structure. In the case of St. John Lateran, while it has been given a veneer of sixteenth and seventeenth century art, at least some of the original Constantinian fabric remains, as do various medieval elements. However, one can be forgiven for not noticing this as it has been substantially covered over in great part. This was in great part due to the fact that the Lateran basilica suffered from neglect during the middle ages, particularly during the period when the popes were residing in Avignon rather than Rome. As a result, it suffered decay and while it was not subject to a total reconstruction as was St. Peter's, much of the original Roman character of the basilica was lost, with most of what you see in the interior today being the work of Borromini. Here, however, is what the original basilica of St. John Lateran is thought to have looked like, dating back to the time of Constantine when it was built in the year 324 A.D.
However, the ancient Constantinian basilica was not the first structure on this site.
Originally there were large houses here belonging to wealthy Romans, some dating back to the Republican period. At least some of these would be replaced by the emperor Septimius Severus to make way for barracks for the imperial horse guard. However, these too would suffer the same fate as some of the former domestic residences during the reign of Constantine when he chose this as the site for the first and oldest Christian basilica in both Rome and indeed the world -- and there was a certain symbolic value to this choice as this same horse guard was defeated by Constantine at the Battle of the Milvian bridge.
The Vatican Museum further details some of the history of the site and gives some further context for what you will find within the excavations:
Conserved underground below St. John Lateran there are the remains of ancient buildings, constructed before the Christian basilica. The term “Lateran” coincides with the name of two figures in imperial Rome: the first, Plautius Lateranus, participated in a plot against the emperor Nero; the second, Sextius Lateranus... was a friend of the emperor Septimius Severus.
During the first imperial age various houses of high social rank were found here, in which it is still possible to admire the paintings, mosaics and marbles (including a staircase with perfectly preserved steps). These dwellings were demolished in 193 A.D. when Septimius Severus decided to build the new barracks for the Equites Singulares, the cavalry unit selected as the emperor’s bodyguards. Their lodgings consisted of small rooms, of which there still remain walls with red paint, sills and door jambs. An area at the centre of the barracks was devoted to the worship of the emperor, or rather the divine part of his person, in accordance with a religious sentiment that was most strongly felt by those who physically protected the Roman emperors.
After the Battle of the Milvian Bridge (312 A.D.) Constantine abolished the Equites Singulares unit and had the basilica of the Saviour built in the area.
The Vatican Museum has provided a number of photos of these excavated structures, of which we have selected a few for your consideration. One will see here sculptural and architectural remnants, decorative pavements and Roman wall paintings (called 'grotesques').
If you find yourself wondering what some of these spaces might have looked like in their heyday, the following reconstruction will give one a sense:
Much like the Scavi beneath St. Peter's, it is possible to visit the Lateran excavations by means of purchasing a ticket. For information on how, please see here. Please note that tickets must be arranged at least 72 hours in advance, however, it would be wise to arrange your tour well before that.
