The Moors' tower

The Clock Tower, or Torre dei Mori (Moors' Tower), is one of the most original Venetian early Reinassance buildings. Its large astronomical clock has been marking the passage of time in Venice for more than five hundred years.
On the rooftop of the tower are two enormous bronze automatons, known as the Moors because of the patina on the metal, made in 1497. The Moors mark the hours by striking the bell that crowns the edifice. The old bearded one, on the right, strikes the bell two minutes early, a symbol of the time that has passed. The young one, on the left, replies two minutes later, marking the time that is yet to come.
The Clock Tower has also a special mechanism which is activated only twice a year, on the Feast of the Epiphany and Ascension Day. On those days, every hour on the hour from the left door on the half circle above the clock emerges a procession of wooden statues representing the Three Kings led by an Angel, proceeding along the mechanized track in front of the Madonna, then reenters the clocktower on the opposite side.
For more information on the Clock Tower, download the museum guide.