These towers date back to the Safavid Period (1502-1736 AD).The pigeon droppings helped to feed the population of Isfahan which consisted of 500,000 people at the time.
These towers are still in use today, however, modern-day chemicals have made the use of these nearly obsolete.
The checkerboard arrangement of the holes allowed the pigeons freedom of movement and kept the amount of building material to a minimum.
Dung was an essential ingredient in the making of gunpowder, which consists of 75 percent potassium nitrate (saltpeter), 10 percent sulfur and 15 percent carbon. Dung is some potent stuff!
The shah's army had no natural source of potassium nitrate, but was able to manufacture it using a mixture of dung, ash, lime and soil. The process of making gunpowder was widely known in the Middle East by 1280.
Sixty-five of these towers are in existence and are protected by the National Heritage list. Even though this is a strange subject to discuss, I found it to be fascinating when I read about it.
Pics are from here