San Galgano Abbey was erected during the early part of the 13th century and is noted as the first pure Gothic church built in Tuscany. It is laid out in a traditional Cistercian plan with the exception of the inclusion of a bell tower. It consists of a cloister, halls, three aisles, carved capitals and several rose windows. The abbey is located a short five minute drive from Monticiano. There is also a walking trail that leads to the abbey from the city.

The bell tower collapsed just before the start of the 19th century and over the years stone and bricks from the structure were used to build other area churches. The cathedral now stands without a roof on the site along with the Hermitage of Montesiepi which serves as the mausoleum of Saint Galgano.

Inside the Rotunda visitors will find the enclosed tomb of the saint and a rock with the sword embedded up to the hilt. This mysterious relic is the subject of much debate as to its origins and meaning. Local legend says that a sinful knight named Galgano was led by the archangel Michael to Montesiepi where he saw a vision of a round temple. When a voice told him to give up his life of sin, Galgano drew his sword and replied that to do so would be as hard as splitting rocks with his sword. He thrust his sword toward a rock and to his surprise it became embedded up to the hilt. Sir Galgano stayed on the hill and lived out his days in poverty.

The legend of the sword in the stone has drawn pilgrims to the site from all around the world. The similarities between this story and that of King Arthur and Excalibur are not lost on the scholars who continue to debate which story came first and how they may be connected.