On the 9th of August 1173 the foundation works of what was the bell tower of the Cathedral began, but which would become one of the most famous monuments in the world thanks to the defect that hangs over it. The works were entrusted to the architect Diotisalvi, the same who had started the construction of the Baptistery. The works were interrupted at the third ring because of the subsidence of the ground. The Tower was built on a soil half compact and half on sediments brought by the river Auser (today Serchio) which flowed nearby.
These problems of stability prevented the continuation of the work until 1275 when the site resumed under the guidance of Giovanni di Simone and Giovanni Pisano, adding three floors to those of the original construction. In an ingenious way: the three added floors tended to curve in the opposite direction, counterbalancing the slope.
The tower was completed in the middle of the following century, adding the belfry. The structure of the monument is very particular because it is made up of two concentric cylinders connected by an internal spiral staircase that leads to the upper cell. On the external cylinder there are six loggetta floors marked by columns with round arches.
The seven bells each have a name and a note: Assunta in B2, Crocifisso in C#3, San Ranieri in D#3, Dal Pozzo in G3, Pasquereccia in G#3, Terza in Bb3, and finally Vespruccio in E4.
The Tower was closed from 1990 to 2000: the long and extremely delicate restoration work avoided its collapse thanks to the intervention of the International Committee of Experts.
Today the Tower is straighter, by 40 cm, the same slope it had about two centuries ago.