The recent archaeological excavations in Piazza della Signoria have furnished evidence that present day Florence was already occupied in prehistoric times. Other signs document the presence of a village in the early iron age and in Etruscan times. But the real foundation of the city dates to Roman times and the oldest part of the city with its network of streets in an orthogonal pattern bears the imprint of these Roman origins. What the earliest chronicles had to say about the origins of the city, albeit in fable form, seems therefore to be based on fact. When it originated as one of Caesar's colonies, the operations involved in founding the castrum and the division of the land into centuriae began in the spring of 59 B.C., at the time of the ludi florales (the probable source of the name Florentia). The colony was laid down following the axis of the consular Via Cassia, which ran along the northern edge of the Florentine basin. For the sake of defense, the city was set at the confluence of two streams (the Arno and the Mugnone) where the oldest populations had previously been located.
Rectangular in plan, it was enclosed in a wall about 1800 meters long. The built-up area, like all the cities founded by the Romans, was characterized by straight roads which crossed at right angles. The two main roads led to four towered gates. The decumanus maximus (at present the streets of the Corso, Speziali and Strozzi) and the cardo maximus (Piazza San Giovanni, Via Roma and Via Calimala) converged on a central square, the forum urbis (now Piazza della Repubblica) where the Curia and the Temple dedicated to the Capitoline Triad (Jupiter, Juno, and Minerva) were later to rise. The topography of the city differed in its orientation from that established by the division of the surrounding land into centuriae, whose axis was, as mentioned, the Via Cassia. The castrum, on the other hand, adhered to the classical ritual of an orientation based on the cardinal points.................


Further information:
www.arca.net/tourism/florence/arthisto.htm