How did all these cities like Genoa, Milan, Venice and Florence, to name just a few, gain and preserve their independence? How was it possible for them when most other population centers across Europe couldn't (at least to that extent), and the general trend seems to have gone toward a certain amount of centralization? Also, how did their political structures develop, and why were they so different from the rest of the continent? And lastly, what changes ultimately lead to the loss of their independence (and why did Venice hold out longer than most of the others)?
Hello there.
While you wait for a fully qualified response, here's a few pieces to get your started.
For Venice, its strength and independence, you can check out these answers
Here's a pretty sizeable answer from /u/manfrin and /u/AlviseFalier. https://www.reddit.com/r/AskHistorians/comments/4fn3r9/italian_history_how_did_the_republic_of_venice/
Here's another nice discussion on the Venetian independence, check out the answer by /u/Guckfuchs.
Here's a more broad-scoped thread discussing something similar to what you're asking, regarding the Italian city states. https://www.reddit.com/r/AskHistorians/comments/14hrj4/how_did_the_small_italian_city_states_survive_and/
As for the political details, kindly check out this thread. https://www.reddit.com/r/AskHistorians/comments/7seycm/the_italian_city_states_during_the_14th_and_15th/
Pinging /u/AlviseFalier
Cheers
Hi there! This is my exact area of interest; and although I'm more than happy to answer additional questions, these older answers can get you started:
I explained the main drivers behind Italian fragmentation in this answer, but if you're interested in more minutiae I also wrote more specific descriptions of the ninth, tenth and eleventh centuries, diving into some of the driving factors behind the emergence of Italian urban republics.
I also wrote more general answers on the medieval Italian political system and wrote comments on the instability of that same system.
Lastly, I wrote an answer on the government of Florence in the Renaissance, as well as a longer answer on the Venetian political system.