Pisa's tower is probably the most famous tower in Italy and maybe in the world and you definitely have to visit it.
But if we talk about the "City of the Towers", well Bologna has a few enemies.
In the picture below from Wikipedia you can see how Bologna looked like in XI century: there were possibly up to 180 towers owned by the richest families of the city to show their prosperity.
Of the numerous towers originally present, fewer than twenty can still be seen today but still you can go up and visit some of that or (in just a few like Torri Prendiparte) have a romantic dinner from the top of the city.
Follow this itinerary not to miss any of the remaining towers or, If you're in a rush and you can't spend more than a few hours in town, climb the 498 wooden stairs to reach the top of Asinelli’s of Torre degli Asinelli to enjoy the beautiful texture of Bologna’s red roofs and admire a full 360° view of the city.
Bologna in the early Middle Ages about 11th century.