With over 50,000 inhabitants, Siena is quite a bit larger than the other medieval towns in Tuscany. Although it also has been preserved well, I found it a lot harder to like (even tacky in some parts). I had to walk for about 20 minutes from one of the parking lots that surround the city center and are aimed at handling loads of tourists (and their buses) in the summertime. Parking at the Stadium is much closer, though not free.
My first glimpse of something grand was at the Baptistry, an enormous white-and-black marble building. It is adjacent to the splendid Duomo. This cathedral has an overly decorated gothic facade, and a striped bell tower almost like a minaret. Its dome is under construction at the moment and covered up. The colours black and white are used all over the design of the Duomo, as they are the colours of Siena's coat of arms.
The shell-shaped Piazza del Campo undoubtedly is the highlight of Siena. The square houses several monumental buildings, like the Palazzo Pubblico. It is also the place where the Palio horse race is run twice a year. I've watched that spectacle on TV once, and it is hard to imagine the crowds and the nervous horses at this so gentle public square. A sight not to be missed there is the 15th century Fonte Gaia, a fountain that is adorned on three sides with bas-reliefs.