The cathedral’s architectural brilliance is remarkable in many ways. Built on an area of 12,000 square meters and weighing 325,000 tonnes, it is the second largest church in the world. Its intricate and elaborate designs are simply astounding. Every inch, including the hidden parts, is decorated with utmost care.

On entering the Cathedral Square, your eyes first rest on the façade. The building of the façade started in 1590 but its construction was marred by many trial and tribulations. Originally planned in the Mannerist-Renaissance style it was supposed to have large doors and lower windows. But the plan was abandoned, as the design was not in keeping with the design of the rest of the building. New design plans were drawn in1650.This design kept the construction of the old design, making changes to the columns and buttresses in the Gothic style. The façade, a mixture of the Mannerist and Gothic styles, was finally completed in 1807-14.

The newly completed façade was not satisfactory and architect Giuseppe Brentano won the redesigning competition for his façade in Gothic style. He did not live to complete the work. In the 20th century it was decided that the façade should be left as it is after only slightly modifying the top row of marble tracery.

The sculptures on exterior were built between the 14th and 20th centuries. They were built from east to west and from bottom to top.

Dedicated to San Giovanni Buono (7th-century bishop of Milan), the colossal altar in the center of the transept was built in the 18th century. It replaced a door that was used until 1568. Agostino Busti sculpted the late 16th-century altar showing the Presentation of the Virgin Mary. You can see this to the left of the present lateral door. To its left is an unusual statue of St Bartholomew. The people of Milan call this statue "the martyr of the taxman". What resulted in this name was the fact that the saint was martyred by being skinned alive. This realistic sculpture is the work of Marco d'Agrate in which depicts St Bartholomew wearing his skin around his neck. You can also see his face and scalp behind his left arm.

The northern sacristy door is another fine piece of Gothic work, late 14th century and the oldest piece of sculpture in the Cathedral. In the northern transept, the altar of the "Madonna dell'albero" is dedicated to Mary, with scenes from her life in the sculpture and in the stained glass. In front of this altar is the large candelabra, the "Candelabro Trivulziano", dating to the late 12th century. This incredible piece of bronze work includes, in the lower part, the twelve signs of the zodiac, the Vices and Virtues, scenes from the Old Testament, the Three Kings on their way to see the Child, and the Virgin and Child enthroned.

To the left of the Madonna dell'albero altar is the altar dedicated to Santa Caterina da Siena. This is the only Gothic altar in the Cathedral, and it was taken from the Basilica of Santa Tecla, a church on the site that was demolished to make room for the Cathedral.

Across the west end of the Cathedral, the brass line on the floor is a sundial that functions with a ray of light entering from above the first window on the right. It was installed in 1786, and restored in 1976, with the addition of the signs of the zodiac that had been removed when reflooring was done earlier.

High above the altar is a red light, which marks the position of a niche that holds the Holy Nail is kept. Believed to be one of the nails with which Christ was crucified, it was brought from the Far East by the mother of Emperor Constantine, and incorporated by the Emperor into the bit of his horse. It was lost but was luckily relocated. It is kept high up to ensure its security. Once a year (on or near the 14th September, feast of the Exaltation of the Cross) it is retrieved from the niche by means of a makeshift lift. This lift was constructed in the 17th century and was then manually operated. Today, it is powered by electricity.

The ceremony of the Holy Nail is the most important ritual at the Duomo of Milan. This relic is believed to be one of the two nails of the Crucifixion discovered by Saint Helena in the Holy Land in 326 A.D. She had one made into a crown and the other into a horse’s bit as a present for her son, the Roman Emperor Constantine. A little later, the then bishop of Milan, Ambrose (now the city's patron saint), received one of the nails from Emperor Theodosius and conserved it in the church of Santa Tecla. Many people believe that the nail came to Milan during the Crusades, in the 12th or 13th century. In 1461, it was lifted to the ceiling of the east end of the Cathedral and placed at a height of 42 meters from the floor.

Saint San Carlo, who instituted the ritual of the Holy Nail in 1576 when the city broke out in plague. The nail was carried through the city in procession and from this day forward the ceremony has been recurring every year in September.

A special Mass is held to bring the Nail down during on the Saturday closest to the Feast of the Exaltation of the Cross in mid-September. The Nail is carried in a procession on Sunday. Earlier it was taken around the Piazza and its neighbouring streets but today the Nail’s journey is confined within the walls of the Cathedral. The Nail goes back to its place on Monday.