Il Duomo ("The Cathedral") is the most famous landmark of Milan and the fifth-largest church in the world. You can’t avoid it if you visit Milan, because it’s in the middle of the city and all main streets radiate from it. The best you can do is climb up to the roof on a sunny day and enjoy the splendor.
All guides and maps about Milan describe the cathedral, so there’s not much extra I can provide here. I’ll point out just a few interesting things.
The building itself is huge with a capacity of 40.000 persons. Rumour has it that all Milanese were supposed to fit inside the church. In 1386, when the first stone was laid, the city population was right about 40.000.
It took almost six centuries to complete the cathedral, as the last gate was inaugurated in 1965. The style is Italian Gothic, but in reality it’s a mixture of contrasting styles. Not a big surprise considering that a huge number of architects, engineers, and artists have been involved during the 600 years of construction.
An important milestone came in 1805 when Napoleon Bonaparte was about to be crowned King of Italy in the cathedral. He gave the order to "speed up things" and surprisingly enough a lot of things were finally completed, for example the facade.
If you want to see the inside of the Cathedral or climb up to the roof, you need to buy tickets. I recommend you do both, at least once. Tickets cost 10-15 euros depending on what you want to do. Check out the latest information here: www.duomomilano.it
My bonus tip for the lazy hedonist is to go to the eighth floor of luxury department store La Rinascente. There you'll find a sun terrace facing Piazza del Duomo and you can order a good Prosecco. Now you'll have the perfect surrounding for enjoying a nice view including the spectacular roof of Il Duomo. Convenient and stylish!