San Clemente

If you want to understand how Rome was literally built on top of itself, there is no better place than San Clemente, a short walk from the Colosseum along Via di San Giovanni in Laterano. At street level is a twelfth-century basilica, still run by Irish Dominicans, with a medieval marble choir enclosure, a cosmatesque floor and one of the most beautiful apse mosaics in the city, the Triumph of the Cross, in which a vine of acanthus spirals out from the cross to embrace birds, animals, saints and ordinary people at work. A chapel off the nave holds Renaissance frescoes by Masolino. Then you go down. A staircase leads to the fourth-century basilica that the upper church replaced, rediscovered in the nineteenth century, its walls painted with rare early medieval frescoes, including the legend of St Clement and an inscription that is often cited as one of the earliest examples of written Italian, a comic exchange in which a pagan orders his servants to drag away a saint. Go down again and you reach the first century: the brick and stone rooms of Roman buildings, including a house and a warehouse or public building, and a Mithraeum, a small shrine to the eastern god Mithras, with its altar showing the god slaying a bull and the benches where his initiates gathered for ritual meals. Throughout the lowest level you hear the rush of water from an ancient underground stream still running through the rock, an eerie reminder of how far below the modern city the Roman streets lay. The upper church is free; the excavations require a modest ticket, and the basilica asks visitors to book entry in advance.

Practical rhythm: allow an hour. Spend fifteen minutes in the upper church with the apse mosaic and the Masolino chapel, then descend with your ticket to the lower basilica and the Roman level. The underground is cool and slightly damp year-round, with narrow passages and steep steps in places, so it is not suitable for wheelchairs or strollers. The basilica's own site handles bookings for the excavations; at busy times slots fill, so reserving ahead is wise. Hours include a midday closure and shorter Sunday hours because the church remains a working parish, so check the schedule before you go. Photography is not allowed in the excavations.

Who it suits: history buffs adore San Clemente, because it explains the city's archaeology in a single building better than any textbook. Families with older children enjoy the sense of descending through time, and the sound of the underground river is a highlight; younger children may find the narrow lower levels less fun. Art lovers come for the apse mosaic and the Masolino frescoes upstairs, which alone justify the walk. As a sacred place, the dress code of covered shoulders and knees applies.

Plan around it: San Clemente sits on a quiet street between the Colosseum and the Lateran, so it slots naturally after a morning at the Colosseum, a ten-minute walk away, or before a visit to the great basilica of St John Lateran, Rome's cathedral, further along the same street. Combine it with lunch in the nearby Monti district or Celio, and with Santa Maria Maggiore for an afternoon of layered churches. For many visitors it becomes the surprise favourite of the trip.