The majestic city walls are one of the most representative examples of medieval and Renaissance military architecture of the Maremma and perhaps of the whole of Tuscany. The building had two distinct phases: an Aldobrandesca phase
and a Sienese phase. The south-east and north sides were constructed
by the Aldobrandeschi counts between the thirteenth and fourteenth centuries. In 1327 a fire caused it a serious damage. In 1373 the Sienese rebuilt most of the walls, completing the south-western side with circular towers and walkways
of the guards in the 15th century.