San Marco Evangelista al Campidoglio

Stand still for a moment and listen. Behind you, the traffic of Piazza Venezia circles the white marble of the Vittoriano without pause. In front of you, a quiet Renaissance loggia opens into shadow. The air cools by a few degrees as you step under its arches. The stone underfoot is worn smooth by centuries of shoes. This is the Basilica of San Marco Evangelista, Saint Mark the Evangelist. It is one of the oldest churches in Rome. Most people walking past have no idea it is here.

That is the first surprise. This church does not stand alone. It is built into the side of Palazzo Venezia, the great fortress-like palace that dominates the square. From the piazza, the basilica can look like just another wing of that building. Look carefully at the facade, though. The open arcade, the second row of arches above, the calm proportions — this is a church front, and a very early Renaissance one in Rome.

The story begins long before the palace. In the fourth century, a pope named Mark founded a church on this spot. He served only a short time, in the 330s, just a generation after Christianity became legal in the Roman Empire. He dedicated the church to his namesake, the Gospel writer Mark. Tradition holds that the relics of Pope Mark himself now rest beneath the high altar. So the church carries two Marks — the evangelist and the pope.

Nothing you see on the surface comes from that first building. Like many Roman churches, San Marco has been rebuilt again and again on the same footprint. Excavations beneath the floor have found traces of earlier structures. Each new church sat on top of the last one. Rome does this everywhere. The city does not clear away its past. It builds on it, layer by layer, until the ground itself rises.

The next great chapter came in the ninth century. Pope Gregory the Fourth rebuilt the church in the 830s. His most precious gift is still here. Walk up the nave and look into the apse, the curved space behind the altar. A gold mosaic fills the half-dome. Christ stands at the centre, raising his hand. Saints stand on either side of him, against a deep blue and gold ground.

Now find the figure at the far left. He holds a small model of a church in his hands. That is Pope Gregory the Fourth, offering this building to Christ. Look at his halo. The other saints have round halos. His is square. In early medieval art, a square halo marked a person who was still alive when the image was made. So this is something close to a portrait, made in his lifetime, almost twelve hundred years ago.

Below the mosaic, a line of lambs walks toward a central lamb. They stand for the apostles and for Christ. It is a common design in early Roman churches, but here the colours are especially clear. Notice how the gold changes as you move. From one angle it looks flat and dull. Take two steps, and it lights up. The small glass pieces, called tesserae, are set at slightly different angles on purpose.

The bell tower outside belongs to a later age. It is a medieval brick tower, with small stacked windows divided by little columns. Towers like this rise all over Rome. They are often called Romanesque, the sturdy style of the eleventh and twelfth centuries, before Gothic arrived. Against the grand palace beside it, the tower looks modest and slightly out of place. That contrast tells you how much older the church is than its neighbour.

The neighbour arrived in the fifteenth century. A Venetian cardinal, Pietro Barbo, was the titular cardinal of this church. He began building a palace here. When he became Pope Paul the Second, the project grew much larger. The palace swallowed the church into its structure. The loggia you walked through, and the second level above it, date from this period. The palace later took its name from Venice, and this basilica became the church of the Venetian community in Rome. That is why the Lion of Saint Mark, the symbol of Venice, appears here.

Look up at the ceiling of the nave. It is a deep coffered ceiling — a grid of sunken square panels, carved in wood and covered in gold. It also comes from the time of Paul the Second. Among the panels you can find his family coat of arms, a lion with a band across it. It is one of the finest Renaissance ceilings in the city. Many visitors walk the whole nave and never raise their eyes.

The walls and columns tell a third story. In the eighteenth century, the interior was redecorated in the Baroque style. Painted stucco, marble cladding and new altars were added along the aisles. So in one room you hold three centuries side by side. A ninth-century mosaic. A fifteenth-century ceiling. Eighteenth-century decoration. None of them erased the others. They simply share the space.

Before you leave, step back outside and walk to the corner of the building. There, against the wall, stands a large, battered stone bust of a woman. Romans call her Madama Lucrezia. She is one of the city's talking statues. For centuries, people hung anonymous verses on statues like her, often mocking popes and powerful families. She has lost much of her face. Locals have given her a long list of legends over the years, and she has outlived all the people she was used to criticise.

Now stand once more under the loggia and look out at the square. The noise returns. Buses, scooters, voices, the bright glare of white marble. Behind you, a pope from the fourth century, a pope from the ninth, and a Venetian pope from the fifteenth all left their mark on one building. Take one last look at the arches as you walk on. Very few people in this square will notice them today.