The hub of the city is Piazza del Duomo, a large, mostly pedestrianized square lorded over by the exaggerated spires of the Duomo, Milan’s cathedral. The piazza was given its present form in 1860 when medieval buildings were demolished to allow grander, unobstructed views of the cathedral, and the Galleria Vittorio Emanuele II was constructed to link the piazza with the showy new opera theatre, La Scala.

Milan’s vast Duomo was begun in 1386 under the Viscontis, but not completed until the finishing touches to the facade were added in 1813. It is characterized by a hotchpotch of styles that range from Gothic to Neoclassical. From the outside at least it’s incredible, notable as much for its strange confection of Baroque and Gothic decoration as its sheer size. The marble, chosen by the Viscontis in preference to the usual material of brick, was brought on specially built canals from the quarries of Candoglia, near Lake Maggiore, and continues to be used in renovation today.

The interior

The interior is striking for its dimension and atmosphere. The five aisles are separated by 52 towering piers, while an almost subterranean half-light filters through the stained-glass windows, lending the marble columns a bone-like hue that led the French writer Suarés to compare the interior to “the hollow of a colossal beast”.

By the entrance, the narrow brass strip embedded in the pavement with the signs of the zodiac alongside is Europe’s largest sundial, laid out in 1786. A beam of light still falls on it through a hole in the ceiling, though changes in the Earth’s axis mean that it’s no longer accurate. To the left of the entrance a door leads down to the remains of a fourth-century Battistero Paleocristiano where the city’s patron saint, Ambrogio, baptized St Augustine in 387 AD.

At the far end of the church, the large crucifix suspended high above the chancel contains the most important of the Duomo’s holy relics – a nail from Christ’s cross, which was also crafted into the bit for the bridle of Emperor Constantine’s horse. The cross is lowered once a year, on September 14, the Feast of the Cross, by a device invented by Leonardo da Vinci.

Close by, beneath the presbytery, the treasury features extravagant silverwork, Byzantine ivory carvings and heavily embroidered vestments. Here, too, is the Duomo’s most surprising exhibit: British artist Mark Wallinger’s haunting video installation Via Dolorosa. Commissioned by the diocese of Milan in a bold attempt to resurrect the role of the Church as a patron of the arts, it comprises a large screen showing the last eighteen minutes of Zeffirelli’s Jesus of Nazareth, with ninety percent of the image blacked out, leaving just a narrow frame visible round the sides. Beside here is the crypt housing the remains of San Carlo Borromeo, the zealous sixteenth-century cardinal who was canonized for his work among the poor of the city, especially during the Plague of 1630. He lies here in a glass coffin, clothed, bejewelled, masked and gloved, wearing a gold crown attributed to Cellini. Borromeo was also responsible for the large altar in the north transept, erected to close off a door that was used by locals as a shortcut to the market.

To the right of the chancel, by the door to the Palazzo Reale, the sixteenth-century statue of St Bartholomew, with his flayed skin thrown like a toga over his shoulder, is one of the church’s more gruesome statues, its veins, muscles and bones sculpted with anatomical accuracy and the draped skin retaining the form of knee, foot, toes and toenails.

The roof

Outside again, from the northeast end of the cathedral you can access the cathedral roof, where you can stroll around the forest of tracery, pinnacles and statues while enjoying fine views of the city and, on clear days, even the Alps. The highlight is the central spire, its lacy marble crowned by a gilded statue of the Madonna – the Madonnina, the city’s guardian – in summer looking out over the rooftop sunbathers.