A bell tower is a tower which contains one or more bells, or which is designed to hold bells, even if it has none. In the European tradition, such a tower most commonly serves as part of a church and contains church bells. Modern bell towers often contain carillons.
Whereas early views of many Italian cities (Siena, Perugia, Florence and Bologna are good examples) show skyline thick with the great stone towers erected by the most powerful families in the twelfth and thirteenth centuries, in medieval Venice only the campanili were prominent landmarks on the horizon.
(Veneto-byzantine palaces, p. 31, the architectural history of venice.)
Purpose, The bell is rung to signify the time, to call people to worship, for special events such as weddings and funerals, or historically to sound a civil defence or fire alarm.