If you were to drop a 5 and 10 pound ball at the same time from the leaning tower of Pisa both the 5 pound ball and the 10 pound ball would hit the ground at the same time.
The reason the 5 and 10 pound balls would reach and hit the ground at the same time is due to the acceleration from gravity being the same for every object that falls at the same height.
The force of gravity acting on an object is independent of it's mass.
Between 1589 and 1592, the Italian scientist Galileo Galilei (then professor of mathematics at the University of Pisa) is said to have dropped "unequal weights of the same material" from the Leaning Tower of Pisa to demonstrate that their time of descent was independent of their mass.
The leaning Tower of Pisa does not fall because its center of gravity has been carefully kept within its base.
In short, this is why the leaning Tower of Pisa does not topple over.
The Leaning Tower of Pisa doesn't fall because its center of gravity is carefully kept within its base.