Whereas the nucleus of Florence was built to a strict Roman city plan of straight streets intersecting at right angles, Siena has all the Tuscan hill-town's haphazard organic forms, straggling up and down a forked ridge of not one but three hilltops.
The center of Florence was built using Roman city plans while Siena has roads going in all directions along the hills.
Siena was destroyed and rebuilt to adhere to Roman city planning.