Santa Maria degli Angeli e dei Martiri site features

From Piazza della Repubblica it does not look like a church, and that is not an accident. There is no facade — just a curved wall of bare Roman brick, scooped inward, with a plain door cut into it. It is one of the original exedras of the bath-house's calidarium, left as brick on purpose, and it gives a passer-by almost nothing. People walk past it to the fountain.

Through the door the temperature drops and the room is enormous. You are standing in the frigidarium and tepidarium of the Baths of Diocletian, and the Roman structure is still holding the roof up: vast cross-vaults springing from monolithic columns of red granite brought from Egypt, so thick you cannot reach round them. The scale is wrong for a church and right for a bath, because that is what it is.

The shape is wrong too. The long arm runs across rather than ahead — over ninety metres of it — so you enter at the side of a hall rather than the foot of a nave, and the altar is away to your right.

Set into the floor is a brass line. It runs for forty-five metres in a band of yellow-white marble, with the signs of the zodiac inlaid along it, and a small hole high in the south wall throws a disc of sunlight that crosses it once a day. It is not an ornament. It was built in 1702 to check whether the Gregorian calendar was right.

Around the walls, enormous altarpieces in coloured marble frames — canvases several metres tall that were moved here from St Peter's — and a scattering of modern bronze doors and sculpture, dark against all that brick and granite.

The meridian line

Forty-five metres of bronze in yellow-white marble, inaugurated 1702, built by Francesco Bianchini for Clement XI to check the Gregorian calendar reform and predict Easter. Tracks the sun through the zodiac. Still works.

The Roman vaults and granite columns

The cross-vaults of the frigidarium and tepidarium, carried on monoliths of granite from Egypt. Fourth-century structure, holding up a sixteenth-century church.

The brick facade

Not a facade: one of the ancient exedras of the calidarium, with its singular concave form in brick. Uncovered in 1911 by demolishing Vanvitelli's front.

The ninety-metre hall

Turismo Roma's figure for the Roman structure Michelangelo kept — and the reason the church is entered from its side.

What Michelangelo did

Adapted a section of the remaining structure to enclose a church, at eighty-six, in the last years of his life.