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
The 1800s saw a LOT of buildings torn down and/or repurposed due to industrialization. Whether it was the factories themselves or the housing for the workers.
And… the first half of the 1900s saw a LOT of Europe need to rebuild for multiple reasons.
Its easy to fixate on something pretty. But understanding the overall scheme, and how it is related to the current (increasingly global) housing crisis is the point of learning from history.