The city of Siena is a medieval delight whose historic centre is deservedly a UNESCO World Heritage site. It is said that during medieval times, Siena’s rivalry with Florence extended to largesse in urban planning, and the effect is divine. The colours that symbolise the city of Siena – black and white – can be seen in the Duomo, which is also noted for having one of the finest mosaic marble floors in Italy. The main square of Siena is the sloping Piazza del Campo. The paving on the square is unique, shaped like the radiating lines of a clamshell. The Palio di Siena is a horse race that occurs twice a year in the Piazza del Campo, on July 2 and August 16. Jockeys take on fiercely tight turns in the Piazza, riding bareback in a madly quick race that lasts under two minutes (see the Palio on Youtube). Focal point of the Piazza del Campo is the Palazzo Pubblico. There are many frescoes to be found inside, including Ambrogio Lorenzetti’s famous frescoes on good and bad government. The Pinacoteca Nazionale (Art Gallery) is located in the Palazzo Buonsignori, and contains a large collection of 13-16th Sienese paintings.