Piazza del Campo

If you can visit around August 17th, the experience will be 20 times better. This is one of the city's biggest festivals and to celebrate it they decorate their streets with medieval flags, the locals are in the streets with scarves with seals of their clans, singing songs of the great festival, and they prepare the square for a bareback horse race. It's a great way to experience medieval Siena. And be prepared to be hot!