The Leaning Tower of Pisa (), or simply, the Tower of Pisa (torre di Pisa ), is the campanile, or freestanding bell tower, of Pisa Cathedral. It is known for its nearly four-degree lean, the result of an unstable foundation. The tower is one of three structures in the Pisa's Cathedral Square (Piazza del Duomo), which includes the cathedral and Pisa Baptistry. The height of the tower is from the ground on the low side and on the high side. The width of the walls at the base is . Its weight is estimated at . The tower has 296 or 294 steps; the seventh floor has two fewer steps on the north-facing staircase. The tower began to lean during construction in the 12th century, due to soft ground which could not properly support the structure's weight. It worsened through the completion of construction in the 14th century. By 1990, the tilt had reached 5.5 degrees. The structure was stabilized by remedial work between 1993 and 2001, which reduced the tilt to 3.97 degrees. Architect There has been controversy surrounding the identity of the architect of the Leaning Tower of Pisa. For many years, the design was attributed to Guglielmo and Bonanno Pisano, a well-known 12th-century resident artist of Pisa, known for his bronze casting, particularly in the Pisa Duomo. Pisano left Pisa in 1185 for Monreale, Sicily, only to come back and die in his home town. 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. A 2001 study seems to indicate Diotisalvi was the original architect, due to the time of construction and affinity with other Diotisalvi works, notably the bell tower of San Nicola and the Baptistery, both in Pisa. Construction Construction of the tower occurred in three stages over 199 years. On 5 January 1172, Donna Berta di Bernardo, a widow and resident of the house of dell'Opera di Santa Maria, bequeathed sixty soldi to the . The sum was then used toward the purchase of a few stones which still form the base of the bell tower. On 9 August 1173, the foundations of the tower were laid. Work on the ground floor of the white marble campanile began on 14 August of the same year during