The Florentine cathedral, Santa Maria del Fiore or simply, Duomo. It’s fifth largest cathedral in Europe, with length of 153 meters. The construction began in 1296 and it was finally finished 173 years later, in 1436. For a breathtaking view of Florence climb all the way to the top. However, the stairs are quite narrow and not great for people who get claustrophobic. I also strongly recommend getting the skip the line passes before your trip because the wait times can get crazy.