Basilica of San Pietro in Vincoli site features

San Pietro in Vincoli is reached by climbing, and the approach is half the character. From via dei Fori Imperiali you walk partway along via Cavour until a staircase opens on the right and takes you up onto the Oppian Hill — so the church arrives suddenly, on a quiet square, above the traffic.

Inside it is wide rather than tall, and the columns are antique. The plan is a nave and two aisles with three apses, divided by antique Doric columns, the aisles covered with cross-vaults. The proportions are early and plain; nothing soars.

Almost everyone turns right, and that is where the crowd is. Michelangelo's Moses, completed in 1515, sits in the right transept — seated, horned, the beard gathered under one hand — set into a wall monument with figures either side.

What it is doing there is the thing to know. The Moses was originally intended as part of a massive 47-statue free-standing funeral monument for Pope Julius II. The scheme was never built, so this is a fragment of an abandoned project, mounted against a wall — which is why a figure conceived to be seen from all sides is now visible only from the front.

The reason the church exists is under the main altar, and it is easy to walk past. The chain said to have held Saint Peter is kept in a reliquary under the main altar; the church was first rebuilt on older foundations in 432–440 to house that relic and is also known as the Basilica Eudoxiana. Since 1894 a link of the chain has been kept in St Peter's. The Canons Regular of the Lateran, who serve the church, observe plainly that most people enter to admire the art and risk forgetting what the place is.

The fabric is a sequence of rebuildings, not one moment. It was consecrated in 439 by Sixtus III and has been reconstructed several times since, including a restoration by Pope Adrian I. The Canons record that the church existed before their arrival and was built in the fourth century.

There is more on the walls than most visitors notice. Two canvases by Guercino — a Saint Augustine and a St Margaret — and the monument of Cardinal Girolamo Agucchi. By the left side of the entrance is the burial of the Florentine painter and sculptor Antonio del Pollaiuolo.

And the building next door is a university faculty. The adjacent block, formerly a convent associated with the church, houses the Faculty of Engineering of La Sapienza — which is why the square outside is full of students rather than tour groups.

Michelangelo's Moses

Completed in 1515 for the tomb of Julius II, and the reason almost everyone comes. It was designed as one part of a free-standing monument of forty-seven statues and ended as the centre of a wall tomb. The horns are a translation. Moses is shown with them to connote the radiance of the Lord, because the Hebrew word for "beams of light" resembles the word for "horns" — a kind of iconographic symbolism common in early sacred art, and, as the sources note drily, horns are easier for a sculptor to carve than rays of light.

The chains of Saint Peter

In a reliquary beneath the main altar: the chains that bound Peter in Jerusalem, given by the Empress Eudoxia to Pope Leo I. By legend, when Leo compared them with the chain from Peter's final imprisonment in the Mamertine Prison in Rome, the two fused together. Since 1894 one link has been in St Peter's Church in Rutland, Vermont.

The tomb of Nicholas of Cusa

The fifteenth-century cardinal and philosopher, died 1464, in a tomb by Andrea Bregno with a relief of Cardinal Nicholas before St Peter.

Domenichino's work

The monument of Cardinal Girolamo Agucchi, which he designed, and the sacristy fresco of the Liberation of St Peter of 1604 — the church's founding story, painted a few metres from the relic itself.

Guercino and Roncalli

Two canvases by Guercino, of Saint Augustine and Saint Margaret, and a Deposition by Cristoforo Roncalli as the altarpiece of the first chapel on the left.

The grave of Antonio del Pollaiuolo

The Florentine painter and sculptor is buried at the left side of the entrance. He is the man who added the figures of Romulus and Remus to the Capitoline Wolf — so the two most famous infants in Rome are his work, and almost nobody stops at his tomb.

The Pontelli portico

The five-arched loggia of 1475 across the front, attributed to Baccio Pontelli, and the cloister of 1493–1503 attributed to Giuliano da Sangallo — now the courtyard of La Sapienza's Faculty of Engineering, which occupies the former convent and carries the same name.