The Tower of Pisa is the bell tower of the Cathedral. Its construction began in the August of 1173 and continued (with two long interruptions) for about two hundred years, in full fidelity to the original project, whose architect is still uncertain. It is situated behind the cathedral and is the third oldest structure in Pisa's Piazza del Duomo (Cathedral Square) after the cathedral and the baptistry.
In the past it was widely believed that the inclination of the Tower was part of the project ever since its beginning, but now we know that it is not so. The Tower was designed to be "vertical" Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 due to a poorly laid foundation and loose substrate that has allowed the foundation to shift direction. The tower presently leans to the southwest.
Both because of its inclination, and its beauty, up to the present the Tower has been the object of very special attention. During its construction efforts were made to halt the incipient inclination through the use of special construction devices; later colums and other damaged parts were substituted in more than one occasion; today, interventions are being carried out within the sub-soil in order to significantly reduce the inclination and to make sure that Tower will have a long life. In this entire story it is possible to find a meaningful constant, the "genetic code" of the Tower: its continual interaction with the soil on which it was built.
Also some history following this construction: Galileo Galilei is said to have dropped two cannon balls of different masses from the tower to demonstrate that their speed of descent was independent of their mass. This is considered an apocryphal tale, and the only source for it comes from Galileo's secretary; During World War II, the Allies discovered that the Nazis were using it as an observation post. A U.S. Army sergeant was briefly entrusted with the fate of the tower and his decision not to call in an artillery strike saved the tower from destruction.
Now, hundreds & thousands of travelers each day go there to view and admire the historic relics, of course they are also deeply attracted by the civilization of ancient Roma.