To understand why the Leaning Tower of Pisa never collapsed, we must look back at its 850-year history, including soft ground, decades of war, and some radical changes in planning.
The Leaning Tower of Pisa is a 57-metre (186 ft) high bell tower located in Pisa Cathedral Square.
The triumphant angle of the Leaning Tower of Pisa is not an optical illusion. In the early 90s of the 20th century, it was tilted at an angle of 5.44 degrees, close to the catastrophic critical value of 5.5 degrees, which would almost certainly lead to the collapse of the structure. Fortunately, engineering work completed in 2008 succeeded in adjusting its tilt angle to 3.99 degrees.
This tilt problem has been around for centuries. The construction of the Leaning Tower of Pisa began in 1173 AD. After only five years of construction work, Pisa's swamp-like soil began to slowly eat away at the foundations on the south side, causing a tilt.
If the engineers simply continued to build and add floors, it would almost certainly have collapsed due to its own weight. Suddenly, construction work was interrupted due to a series of wars that broke out between the Republic of Pisa and Genoa, Lucca and Florence.
Construction was not resumed until 1272 AD, when the soil had slightly solidified, allowing the builders to raise it again. However, as the floors increased, the architects realized that the inclination was intensifying and took steps to adjust the original plan to make the south side of the tower slightly higher.
This is evidenced by the difference in the number of steps found on the top floor: six on the south side and four on the north side. This also means that the structure is slightly curved, although it is barely visible to the naked eye.
However, by this time it was already too late. The tower has acquired its signature tilt, which has kept city planners awake at night for centuries. When the 20th century came, engineers rushed to solve the problem, but some of their attempts were more successful than others.
The low-key nature of these efforts can be seen in the wonderful TED-Ed video above.