Although Milan is rarely mentioned in the same breath as other Italian cities that wear their cultural heritage on their sleeves, it still has a few tricks up its own worth checking out. One of the biggest cathedrals on Earth, construction on the Milan Cathedral began in the 14th century and was not be fully completed until the 20th century. All the work was not for naught, for it is the most large and extravagant church in all of Italy (personal views on its aesthetic aside).
Most notable about the exterior is the front facade, famous for its five bronze doors and its Gothic steeples. Inside, it's a bit dark, but breathtaking in its use of such a cavernous space. The views from the roof, which you can get to for a nominal fee, are also excellent and worth doing as part of your tour of the grounds. Also, as you'd probably suspect, the cathedral is the centerpiece of the buzzing Piazza del Duomo in Centro Storico, meaning that there will be no area in Milan more packed with tourists. And why not? If there is one landmark in the city that everyone needs to see, it's undoubtedly the Milan Cathedral.