These mini castles were built all over the city.
The reasons for the construction of so many towers are not clear. One hypothesis is that the richest families used them for offensive/defensive purposes during the period of the Investiture Controversy.
Only a few survive to the current day.
https://en.m.wikipedia.org/wiki/Towers_of_Bologna https://www.sciencedirect.com/science/article/pii/S1296207418306952
But their plans were foiled because Ubisoft thought its all baloney