The Founding of Florence
In the 3rd century BC, that Latin city on the Tiber called Rome began its expansion, and some of the first neighboring peoples to fall were the Etruscans. Some cities, such as Perugia and Arezzo, allied themselves with Rome and were merely absorbed, while others, such as Volterra and Orvieto, were conquered outright.
The Romans brought with them the combination of a highly conservative (some say stagnant) culture coupled with a certain degree of adaptiveness to local religious beliefs and social modes that allowed their empire to stay solvent for so long. As the Romans gained power over the entire peninsula, the removal of political barriers and the construction of roads allowed trade to develop and flow relatively uninhibited.
Many of the old cities flourished, and the general prosperity led to the founding of new cities throughout the region, especially as retirement camps for Roman soldiers. While the Etruscans were keen on building cities on hilltops—both for easy defensibility and to leave more valley land free for farming—the Romans preferred flat ground along rivers, which allowed them to lay out exacting street-plan grids oriented precisely to the points of the compass. They were fanatically single-minded about city planning, rarely wavering from this model (which is also why Flroence's grid of streets doesn't really quite align parallel to the Arno, since the Arno inconveneivlty does not flow dues east-west.).
During the lull of the later Roman Empire, Christianity quickly spread throughout much of central Italy—Lucca even claims to have converted in the 1st century a.d. through the efforts of one of St. Peter’s own followers, though Pisa tries to one-up its neighbors by claiming a church first built by St. Peter.