As in nearby Pisa, Sienna, and Florence, the wealthy landowners began building a tower onto their palace home, both as an overt manifestation of their wealth and power, but also as a defensive position and lookout when things got rough. In battles, destruction of the towers - often specifically targeted properties of certain families - was common, and a blow to the pride of the victimized town thereafter.
Restored and reopened in the 1980s, today the tower’s rooftop garden is accessible via a climb of 230 stairs (a modern addition - the original stairs were on the outside). From the rooftop you can see the outlines of the three concentric city walls, the Roman center of town, and the few remaining other towers. Across town, the Torre della Ore (clock tower) is also open for climbing.
According to atlasobscura.com