The Milan Cathedral has long been the single strongest iconic image of both Milan and Lombardy. Incredibly, it took five hundred years to complete its construction. The history of it's construction is so long, and the architecture so complex, that I will not put the entire Wikipedia page here.
From Wikipedia -- Milan Cathedral (Italian: Duomo di Milano; Milanese: Domm de Milan) is the cathedral church of Milan in Lombardy, northern Italy. Dedicated to Santa Maria Nascente (Saint Mary Nascent), it is the seat of the Archbishop of Milan, currently Cardinal Dionigi Tettamanzi.
The Gothic cathedral took five centuries to complete. It is the largest Gothic cathedral and the second largest Catholic cathedral in the world.
The American writer and journalist Mark Twain visited Milan in the summer of 1867. He dedicated chapter 18 of Innocents Abroad to the Milan Cathedral, including many physical and historical details, and a now uncommon visit to the roof. He describes the Duomo as follows:
"What a wonder it is! So grand, so solemn, so vast! And yet so delicate, so airy, so graceful! A very world of solid weight, and yet it seems ...a delusion of frostwork that might vanish with a breath!... The central one of its five great doors is bordered with a bas-relief of birds and fruits and beasts and insects, which have been so ingeniously carved out of the marble that they seem like living creatures-- and the figures are so numerous and the design so complex, that one might study it a week without exhausting its interest...everywhere that a niche or a perch can be found about the enormous building, from summit to base, there is a marble statue, and every statue is a study in itself...Away above, on the lofty roof, rank on rank of carved and fretted spires spring high in the air, and through their rich tracery one sees the sky beyond. ... (Up on) the roof...springing from its broad marble flagstones, were the long files of spires, looking very tall close at hand, but diminishing in the distance...We could see, now, that the statue on the top of each was the size of a large man, though they all looked like dolls from the street... They say that the Cathedral of Milan is second only to St. Peter's at Rome. I cannot understand how it can be second to anything made by human hands."