The story of the tower goes a little like this: way back in the time of the Crusades, the Count of Tessé found that his horse was no longer fit for battle once returning from the Holy Wars. He decided to set him free in the forest but was greatly surprised when the animal returned to his stable rejuvenated, strong and lively a short time afterwards. The Count decided to follow his hoofprints to a warmstream now known as the Grande Source and bathed in the water. He was amazed at its effects, and apparently went off and fathered several children with the Dame de Bonvouloir.
In the 13th century, Lord Guyon Essirard heard the story and went to bathe in the stream. He then had ten children and was so happy with himself that he built the Tour de Bonvouloir to show his gratitude. Some even say that the tower is in the form of a phallus!