Short answers, pulled from the story.
The year 753 BC marks the founding of Rome, a city-state that would eventually create colonies and municipi across Italy. Earlier examples existed in Magna Graecia and Etruria where urban settlements thrived before Roman expansion.
The Lombard League formed an alliance at its apex with most northern Italian cities such as Milan, Piacenza, Cremona, Mantua, Crema, Bergamo, Brescia, Bologna, Padua, Treviso, Vicenza, Verona, Lodi, Reggio Emilia, and Parma. Membership changed through time but remained a powerful coalition against Holy Roman Empire rule.
Venice conquered three-eighths of the Byzantine Empire during the Fourth Crusade in 1204. The Republic of Venice expanded to conquer territories on mainland Italy known as Domini di Terraferma and abroad as Stato da Màr while maintaining important trade communications with the Muslim and Byzantine world.
Luca Pacioli created the banking system of the Italian city-states with double-entry bookkeeping. His 27-page treatise on bookkeeping contained the first known published work on that topic which laid the foundation for double-entry bookkeeping as practiced today by Genoese merchants.
Gian Galeazzo Visconti bought the title of Duke of Milan from King Wenceslaus in 1395 for 100,000 gold florins. This shift transformed political structures across the peninsula where most republics had become princely states dominated by a Signore by 1300.