Palazzo Pubblico, which was designed also to provide consolidation for the supporting wall between Piazza del Campo and Piazza del Mercato Vecchio below it, was built in various stages. It possess an extraordinarily valuable historic-artistic heritage, with works such as the Maestà of Simone Martini and the Good and Bad Government of Ambrogio Lorenzetti, as well as Italian and foreign paintings dating from the 16th to the 18th century, ceramics of Sienese manufacture dating from the 14th to the 18th century, and other precious collections.

In the Council Hall was placed the revolving Globe of Ambrogio Lorenzetti. Consisting of a round revolving frame covered in lightweight material, it was painted by the famous Sienese artist in 1345 but was lost a few centuries later. Unfortunately, no satisfactory descriptions of this instrument have survived. It is presumed, however, that it was a real topographic chart in which the Sienese State was symbolised as the centre of a constellation of satellites represented by the surrounding country and, in the background, by the other known lands.