Due to the unique structure of the Round Baptistry, the acoustics provide one of the most amazing effects...
In fact, all of the buildings in this church complex have suffered from unstable ground. The main cathedral (Duomo) has also sunk slightly from one corner. The height of the tower is 55.86 metres (183.27 feet) from the ground on the low side and 56.67 metres (185.93 feet) on the high side. The width of the walls at the base is 2.44 m (8 ft 0.06 in). Its weight is estimated at 14,500 metric tons (16,000 short tons). The tower has 296 or 294 steps (no, we did not have time to climb); the seventh floor has two fewer steps on the north-facing staircase. Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees. This means that the top of the tower is displaced horizontally 3.9 metres (12 ft 10 in) from the center
Construction of the tower occurred in three stages over 199 years. 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, because the Republic of Pisa was almost continually engaged in battles with Genoa, Lucca, and Florence. This allowed time for the underlying soil to settle. Otherwise, the tower would almost certainly have toppled. In 1198, clocks were temporarily installed on the third floor of the unfinished construction.
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 (like a banana). 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. The bell-chamber was finally added in 1372. There are seven bells, one for each note of the musical major scale. The largest one was installed in 1655.
After a phase (1990–2001) of structural strengthening, the tower is currently undergoing gradual surface restoration, in order to repair visible damage, mostly corrosion and blackening.
Everyone tries to correct the lean, few succeed.