Detached from the Cathedral, the sumptuous and characteristic bell tower stands out, also dating back to the 11th century. The cylindrical shape (which recalls the influences of Ravenna), surmounted by a conical spire, makes it unique in the world for its architectural structure and historical age.
The structure probably rose above the base of one of the towers of the city's defensive walls. Evidence of this is the Istrian stone base, which can still be admired today, interrupted only on the north-facing side. Probably born as a watchtower, given its privileged position with respect to the sea, it is 42 meters high and the harmonious alternation of mullioned windows, mullioned windows and small columns makes it a splendid example of Gothic style.
The cylindrical barrel, 28 meters high, is divided into seven floors, with an average radius of approximately 3.3 metres. The cusp, supported by a cylindrical nut 1.5 meters high and with a radius of 2.8 metres, is 10 meters high and is surmounted by an iron cross with a wind flag, approximately 2.5 meters high. At the second register there was a clock; it was removed during restorations at the beginning of the 20th century. Internally the registers are now divided by reinforced concrete floors (with the exception of the second, still made of wood), created as part of recent consolidation works and connected by wooden stairs; in the last register there are three bronze bells.
The bell tower is a leaning tower: it is in fact inclined by approximately 1.4º in the East-South-East direction (approximately 1/3 of the inclination of that of Pisa).