On 9th August 1173, work began on the foundations of the bell tower for the Cathedral of S. Maria. It was to be some distance from the church, about 56 metres tall, weighing 14.453 metric tons. Initially, the director of works was probably the architect Diotisalvi, the same person who began the Baptistery. Work stopped after the third ring because of subsidence under the foundations of the Tower. Soft ground and the fact that in ancient times a bend in the river Auser flowed here, making the terrain unstable, is the reason why the tower leans, making it famous.

The problem of instability brought work to an end for a long time.


Building recommenced in 1275 under the guidance of Giovanni di Simone and Giovanni Pisano; three more rings were added to the three already completed. In an attempt to straighten the tower, the three additional floors bend slightly in the opposite direction from the slant of the Tower. It was completed in mid 14th century when the belfry was added.


The structure if the Tower is highly unusual. It is made of two concentric cylinders joined together by a spiral staircase to the topmost story. Externally there are six rings of round-arched galleries. The marble ditch at the base of the Tower was added in the 19th century.


The Tower now slopes at an angle of 5° from the vertical. It remains standing because its centre of gravity is inside the perimeter of its base.


The visit of the tower requires the visitor a considerable physical effort, taking into account that the scale is composed of about 300 steps, so the visit is strongly not recommended for people with heart problems, and in general those who do not enjoy good health.