You can see here the Basilica and the Bell Tower in San Marco Square. It’s possible to go to the top of this bell tower, from where you can see a fantastic view of the city (if the day is clear).
Another famous tower in this square is the Torre dell’Orologio (=clock tower). This tower was constructed at the end of the 15th century. The clock is a work of Giampaolo and Giancarlo Ranieri (14th century). The clock marks the 24 hours (not 12), the season, the moon phases and the zodiac.