From the World Heritage inscription:
Siena is the embodiment of a medieval city. Its inhabitants pursued their rivalry with Florence right into the area of urban planning. Throughout the centuries, they preserved their city’s Gothic appearance, acquired between the 12th and 15th centuries. During this period the work of Duccio, the Lorenzetti brothers and Simone Martini was to influence the course of Italian and, more broadly, European art. The whole city of Siena, built around the Piazza del Campo, was devised as a work of art that blends into the surrounding landscape.
Siena is an easy train ride from Florence. I spent the day there taking photos and strolling around the city. While it is definitely touristy, it is a far cry from the tourist levels you will see in Florence or Venice. In addition to the center of town, the cathedral and side streets are also worth exploring.