It is believed that the Asinelli Tower initially had a height of ca. 70 m and was raised only later to the current 97.2 m (with an overhanging battlement of 2.2 m). In the 14th century the city became its owner and used it as a prison and small stronghold. During this period a wooden construction was added around the tower at a height of 30 m above ground, which was connected with an aerial footbridge (later destroyed during a fire in 1398) to the Garisenda Tower. Its addition is attributed to Giovanni Visconti, Duke of Milan, who allegedly wanted to use it to control the turbulent Mercato di Mezzo (today via Rizzoli) and suppress possible revolts. The Visconti had become the rulers of Bologna after the decline of the Signoria of the Pepoli family, but were rather unpopular in the city.
Severe damage was caused by lightning that often resulted in small fires and collapses, and only in 1824 was a lightning rod installed. The tower survived, however, at least two documented large fires: the first in 1185 was due to arson and the second one in 1398 has already been mentioned above.
The Asinelli Tower 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: During bombing attacks, four volunteers took post at the top to direct rescue operations to places hit by Allied bombs. Later, a RAI television relay was installed on top. During the 1960s, architect Minoru Yamasaki used the Towers as inspiration for his work on the World Trade Center.
The Garisenda Tower today has a height of 48 m with an overhang of 3.2 m. Initially it was approximately 60 m high, but had to be lowered in the 14th century due to a yielding of the ground which left it slanting and dangerous. In the early 15th century, the tower was bought by the Arte dei Drappieri, which remained the sole owner until the Garisenda became municipal property at the end of the 19th century.
It was cited several times by Dante in the Divine Comedy and The Rime (a confirmation of his stay in Bologna), and by Goethe in his Italian Journey. The Two Towers have also been subject of an eponymous poem by Giosuè Carducci as part of the Barbarian Odes.