San Nicola in Carcere

Stand still for a moment and look at the side wall of this church. Grey stone columns are pressed into the plaster, half-swallowed by later masonry, as if the building grew around them. Behind you, traffic hums along the road toward the Tiber. Ahead, a pale facade catches the light. This small church holds more layers of Rome than almost any building of its size in the city.

You are standing at San Nicola in Carcere, a church dedicated to Saint Nicholas, the fourth-century bishop of Myra in what is now Turkey. Yes, the same saint who later became Santa Claus in northern Europe. But the story of this site begins long before Nicholas, long before Christianity, in the days of the Roman Republic.

In ancient times, this ground was part of the Forum Holitorium, the vegetable market of Rome. It lay close to the river, near the old harbour, where barges brought produce into the city. Picture stalls of cabbages, onions and herbs, the shouts of traders, the smell of damp earth and river water. Among the market stalls stood a row of three small temples, side by side.

Those three temples are the reason this church looks so strange. Scholars generally identify them with three deities: Janus, the two-faced god of doorways and beginnings; Juno Sospita, a protective form of the goddess Juno; and Spes, the personification of Hope. The exact matching of each temple to each god is still debated. What is certain is that all three stood here, facing the same direction, during the Republican period.

When Christians built a church on this spot in the early Middle Ages, they did something very practical. They did not clear the ruins away. Instead, they used the central temple as a foundation and reused the columns of its neighbours as ready-made walls. So the building you see is not a church next to ancient temples. It is a church made partly out of them.

Walk along the outside and look closely. On one side, you can see a line of columns set directly into the church wall. Some are of travertine, the creamy limestone used across Rome. Others are of peperino, a darker grey volcanic stone with a speckled surface. The different stones and column styles help archaeologists tell the original temples apart.

Now, the name. In Carcere means in the prison. Here the line between history and legend becomes thin. In the Middle Ages, people believed that a prison had once stood on this site, and some linked the vaults beneath the temples to that idea. Ancient writers also placed a Roman prison somewhere in this neighbourhood, though its exact position is uncertain.

That ancient prison is tied to one of Rome's most touching tales. According to the story told by ancient authors, an old man was condemned to die of starvation in a cell here. His daughter visited him each day and secretly fed him from her own breast. When the guards discovered this, the authorities were so moved that they freed him. Romans called the tale an example of pietas, which means devotion to family and duty. Treat it as legend, not documented fact.

Lift your eyes to the front of the church. The facade dates from the late sixteenth century and is attributed to the architect Giacomo della Porta, who also worked on the dome of Saint Peter's. It is restrained and elegant, with clean pilasters and a simple pediment. Notice how its calm, ordered lines sit over a building whose bones are two thousand years older.

To the side rises a squat bell tower. It appears to have begun life as a medieval fortified tower, one of the many that noble families built across Rome when the city was divided by rival clans. Later it was adapted for bells. So in a single glance, you can see the Republic, the medieval city of towers, and the Renaissance, all stacked together.

The open space around you is itself a story. In the 1930s, the government cleared away many of the houses and lanes that once crowded this area, in order to expose the ancient monuments. That is why the church now stands rather exposed beside a wide road. Before those works, it was hidden inside a dense neighbourhood, and visitors would have reached it through narrow streets.

If the underground area is open during your visit, it is well worth going below. A separate ticket is usually needed. Down there, you walk among the foundations of the temples, on the podiums that once raised them above the market. The air is cool and still. You can see massive blocks of stone and the joins where Roman builders fitted them together. It is one of the few places in Rome where you can stand inside a Republican temple's base.

When you step back outside, look around at the neighbours. Just along the road stand the curved arches of the Theatre of Marcellus, a theatre from the age of Augustus that later became a palace. Toward the river is Tiber Island. A short walk south brings you to the old cattle market area and the church of Santa Maria in Cosmedin. This whole stretch was the commercial heart of early Rome.

Before you move on, stand at the corner and look once more at that side wall. Those grey columns have watched market traders, priests of Janus, medieval monks and modern commuters. They are still doing the same job: holding a building up. As you walk away, listen to the traffic and the footsteps, and remember that this was always a busy crossing point. The sound has changed. The place has not stopped being used.