Siena has preserved its medieval city centre enclosed by mighty walls. This city is situated in a splendid territory and is surrounded by hills covered with vineyards and olive groves, farmhouses, villages, churches and castles scattered in the beautiful Chianti countryside. Of Etruscan origin, Siena became a Roman colony at the time of Emperor Augustus. In the Middle Ages, being the centre of important trade routes that led to Rome, it became increasingly important, reaching its economic and cultural peak and enriching of its most beautiful works of art in the thirteenth and fourteenth century. The end of the independence of the free city of Siena dates back to 1555, when the city was subjugated by Florence. The Cathedral (begun in the twelfth century) and the shell-shaped Piazza del Campo, where it is possible to admire the Palazzo Pubblico (the most grandiose Tuscan Gothic building, built between 1297 and 1342) and the Torre del Mangia, are among the principal monuments of the city. In Siena also worked painters such as Duccio di Buoninsegna, Ambrogio and Pietro Lorenzetti and Simone Martini, many of whose most significant works are preserved in the prestigious museums that the city can boast. Siena also has a deep cultural tradition, as evidenced by its prestigious University and by the international institutions located in here. Siena has also evocative traditions (like the Palio delle Contrade, held July 2 and August 16 of each year), deeply rooted in the city's past, that are now famous around the world and that have become a distinctive feature of its identity.