Standing proudly on the piazza del Duomo, the third largest church in Christendom (outdone only by St Peter's in Rome and Seville's cathedral), the Duomo is truly a joy to behold. Although the key elements were in place by 1391, the Duomo took the best part of 500 years to complete - and indeed, building work continues today: a five-year project to clean the façade started in 2002, and the Duomo's full mind-blowing beauty is now there for all to behold.
The Duomo was begun in brick, but upgraded to marble as its architects understood the grandeur of the project. Over time, it was adorned with Gothic spires and an astonishing wealth of statues, and has been adored by a huge number of art and architecture aficionados. As generations of Lombard builders and architects argued with French and German master stone-cutters about the best way to tackle their mammoth task, an enormous array of styles was employed.
Construction began in 1386 by order of Bishop Antonio da Saluzzo, on a site that had been associated with places of worship since the third century: a Roman temple to the goddess Minerva once stood here. On the orders of Gian Galeazzo Visconti, then ruler of Milan, Lombardian terracotta stone was eschewed in favour of Condoglian marble shipped from Lake Maggiore on the Ticino river, and then along the Navigli, a network of canals in southern Milan built specially for the purpose.
Although consecrated in 1418, the cathedral remained incomplete for centuries. Politics, physical setbacks (a pink granite column sank, in transit, in Lake Maggiore), a lack of money and downright indifference kept the project on permanent standby. Finally, early in the 19th century, the façade was put on the church by order of none other than Napoleon; he kick-started the final stages of construction before crowning himself king of Italy here in 1805.
A staggering 3,500 statues adorn the Duomo, about two thirds of them on the exterior. The oldest are at the apse end, which was built from 1386 to 1447; those along the sides were added as the building work progressed, between the late 15th and early 18th centuries. The façade is baroque up to the first order of windows, and neo-Gothic above. Each of the five bronze doors that provide access to the Duomo was sculpted by a different artist between 1840 and 1965, along particular themes.
To appreciate the statues and 135 spires fully, take the lift (which is near the back of the Duomo, on the left-hand side) to the roof, from where, on clear days, you also get breathtaking views of the Alps. A roof visit brings you closer to an icon dear to the hearts of the Milanese: the Madonnina (1774), the gilded copper figure of Mary on the church's highest spire that was the city's highest point until it was pipped by the Pirelli skyscraper in 1958.
The 52 pillars of the five-aisled Duomo correspond to the weeks of the year. On their capitals, imposing statues of saints stretch up into the cross vaults of the ceiling, to vertiginous effect. On the floor near the main entrance is a sundial installed in 1768 by astronomers from the Accademia di Brera, placed so that it is struck by a ray of sunlight breaking through a hole in the opposite wall. On the summer solstice (21 June), the ray strikes the tongue of bronze set in the floor; on the winter solstice (21 December), the ray of light stretches out to reach the meridian. The sundial is so precise that it was used in the past to regulate clocks throughout the city.
In the first chapel on the right is the 11th-century sarcophagus of Bishop Ariberto d'Intimiano, and a 17th-century plaque commemorating the founding of the Duomo. The oldest of the stained-glass windows in the next three chapels was made in 1470-75 and is in the fifth bay on the right; it shows scenes from the life of Christ.
In the crossing of the transept, the presbytery floor has been worn by the passage of the many millions of pilgrims who have visited the Duomo over the centuries; Cardinal (later saint) Carlo Borromeo wanted the Duomo to serve as his model Counter-Reformation church. Flanking the 15th-century high altar are two gilded copper pulpits, both 16th-century; the organ is also here, its wooden shutters painted with biblical scenes by Giovanni Ambrogio Figino, Camillo Procaccini and Giuseppe Meda.
A nail allegedly from the Cross hangs at the apex of the apse's vaulted roof. Once a year, on the Saturday closest to 14 September (prior to the beginning of vespers), the archbishop ascends to the apex to retrieve the nail, moving up slowly and solemnly through the air in the Duomo's decorated wooden nivola - an angel-studded basket constructed in 1577 under orders by Borromeo, and significantly renovated and redecorated in 1701 (when the putti, or angels, were added). The nail is then exhibited at the altar until the following Monday after vespers, when it's lifted back up to the church ceiling.
In the right transept you'll find a funerary monument to Gian Giacomo Medici, long attributed to Michelangelo but now recognised as the work of sculptor and collector Leone Leoni (1560-63).
On a pedestal in the wall opposite the Medici monument stands an arresting and remarkably lifelike statue of a flayed St Bartholomew. This incredibly accurate study of human anatomy was carved in 1562 by Marco d'Agrate, a student of Leonardo da Vinci. Above and to the right, the splendid stained glass showing St Catherine of Alexandria - who died on the original Catherine wheel - is the work of the Arcimboldo brothers (1556).
Completed in 1614, the sculpture that closes the choir - designed by Pellegrino Tibaldi and carved by Paolo de' Gazzi, Virgilio del Conte and the Taurini brothers - is a masterpiece of its time. The three tiers of the sculpture represent (above) the life of St Ambrose, (centre) the martyred saints venerated by the Milanese church, and (below) the Milanese bishops Anatalone and Galdino.
The ambulatory windows blaze with fabulous 19th-century stained glass by the Bertini brothers, and depict scenes from both Testaments. From the ambulatory, stairs lead down to the crypt, where Carlo Borromeo is buried. Entrances to the treasury and the choir are also in the ambulatory.
In the left transept, the fantastic monsters on the bronze Trivulzio Candelabra, an impressive example of medieval goldsmithing, represent the arts, professions and virtues, and were created by the great 12th-century goldsmith Nicolas of Verdun. In the left aisle, the Cappella del Crocifisso (third past the transept) has stunning 16th-century stained glass.
The remains of the earlier churches of Santa Tecla and the Baptistery (where St Ambrose baptised St Augustine in 387) can be reached by descending the stairs just to the left of the main entrance. The stairs to the right lead to a set of early Christian excavations, two storeys below the Duomo's main door.
From 4 November until the Epiphany, the great Quadroni di San Carlo, a devotional pictorial cycle with scenes from the life of the saint, are displayed in the naves between the pillars. The works are a compendium of 17th-century Lombard painting.