The highets tower of the Two towers was used by the scientists Giovanni Battista Riccioli (in 1640) and Giovanni Battista Guglielmini (in the following century) for experiments to study the motion of heavy bodies and the earth rotation. In World War II, between 1943 and 1945, it was used as a sight post. The Two Towers, both of them leaning, are the symbol of the city of Bologna. They are located at the intersection of the roads that lead to the five gates of the old ring wall (mura dei torresotti). Their construction probably took place between 1109 and 1119. It is believed that the tallest tower initially had a height of ca. 70 m and was raised only later to the current 97.2 m (with an overhanging rock of 2.2 m). In the 14th century the city became its owner and used it as prison and small stronghold.