The Town Hall, of which the original project’s author is unknown, has suffered various constructive phases: in 1304 the opposite part to the Campo was probably already realised, then they proceeded with an “extension” that consisted in the central section that holds the Loggia dei Nove, the Counsel Room, the Offices of Biccherna; the most important changes afterwards were those of Carlo Fontana at the end of the XVII century, regarding the elevation of the two lateral wings, and those at the end of the 19th century to restore the façade in a neo-Gothic sense.
With the creation of the Civic Museum, in the 1930s, the Palazzo reached the structure that it has still today. It has an historical-artistic patrimony of extraordinary value owed to works such as the Maestà of Simone Martini, the Buono e Cattivo Governo by Ambrogio Lorenzetti, as well as Italian and foreign paintings dated to between the XVI and XVIII centuries, ceramics of Sienese manufacture from between the XIV and XVIII centuries and other precious collections.
The Counsel Room once held the turnable World Map painted by Ambrogio Lorenzetti in 1345, consisting in a round, turnable frame covered with a light material, that was finally lost. On the left-hand side of the Town Hall rises the Mangia Tower that, with its 88 metres in height, symbolised the civic liberty and the liberation from feudal power, but that rang for the first time in connection with the terrible plague that unmistakeably damaged the Sienese economy putting an end to its Golden Age.