Over time, the tower has become one of the most visited tourist attractions in the world as well an architectural icon of Italy, receiving over 5 million visitors each year.

The tower began to lean during construction in the 12th century, due to soft ground which could not properly support the structure's weight.

A piece of cast bearing his name was discovered at the foot of the tower in 1820, but this may be related to the bronze door in the façade of the cathedral that was destroyed in 1595.

[11] Work on the ground floor of the white marble campanile began on 14 August of the same year during a period of military success and prosperity.

[12] Nearly four centuries later Giorgio Vasari wrote: "Guglielmo, according to what is being said, in the year 1174, together with sculptor Bonanno, laid the foundations of the bell tower of the cathedral in Pisa".

Construction was subsequently halted for the better part of a century, as the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence.

[16] On 12 April 1264, the master builder Giovanni di Simone, architect of the Camposanto, and 23 workers went to the mountains close to Pisa to cut marble.

It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the belfry with the Romanesque style of the tower.

Leon Weckstein, a U.S. Army sergeant sent to confirm the presence of German troops in the tower, was impressed by the beauty of the cathedral and its campanile, and thus refrained from ordering an artillery strike, sparing it from destruction.

It was, however, considered important to retain the current tilt, due to the role that this element played in promoting the tourism industry of Pisa.

[29] The tower and the neighbouring cathedral, baptistery, and cemetery are included in the Piazza del Duomo UNESCO World Heritage Site, which was declared in 1987.

[32][33] The bells were removed to relieve some weight, and cables were cinched around the third level and anchored several hundred meters away, and residences in the path of a potential collapse were vacated.

[34] After a phase (1990–2001) of structural strengthening,[35] the tower has been undergoing gradual surface restoration to repair visible damage, mostly corrosion and blackening.

However, this bell is older than the bell-chamber itself, and comes from the tower Vergata in Palazzo Pretorio in Pisa, where it was called La Giustizia (The Justice).

[citation needed] The circular shape and great height of the campanile were unusual for their time, and the crowning belfry is stylistically distinct from the rest of the construction.