The tower began to sink after construction had progressed to the second floor in 1178. This was due to a mere three-metre foundation, set in weak, unstable subsoil, a design that was flawed from the beginning . Construction was subsequently halted for almost a century.In 1272 construction resumed under Giovanni di Simone, architect of the Camposanto. In an effort to compensate for the tilt, the engineers built upper floors with one side taller than the other. Because of this, the tower is actually curved. Construction was halted again in 1284, when the Pisans were defeated by the Genoans in the Battle of Meloria.
The seventh floor was completed in 1319. It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the bell-chamber with the Romanesque style of the tower. There are seven bells, one for each note of the musical major scale. The largest one was installed in 1655. The bell-chamber was finally added in 1372. The total amount of years that the building of the tower took was between 185 and 195 years.