It is a magnificent one-nave Romanesque church in which light barely filters through the slits of the well-designed single-lancet windows in the travertine.
The Church of Santa Maria Assunta, was built on land donated in 1016 to the Amiata Abbey of San Salvatore.
Pope Benedict VIII, on May 28, 1017, granted approval for the construction of a church within the castle walls. The church took the name “Santa Maria Assunta in cielo,” which belonged to the PP: Cistercians of the Abbadia. The building at the end of the village of San Quirico was also called “S. Maria ad Hortos” because it was surrounded by vegetable gardens that would later be part of the Horti Leonini.
Underlining “Ecclesia,” the people call it Sante Marie (Ecclesia Sanctae Mariae). And it is named in a Bull issued from the Lateran Palace, Rome by Clement III and bears the date of April 20, 1189 and is addressed to the Bishop Buono of Siena. The Pope in confirming to Buono the benefits and Churches due to him directly mentions the ” Ecclesia Sanctae Mariae que est in burgo Sancti Quirici,” with the recommendation to take it under his protection.
In the disputes between Siena and Arezzo over the boundaries of several parishes, while the Pieve di San Quirico in Osenna, at the center of alternating events, was assigned now to one and now to the other diocese, Santa Maria Assunta always remained with Siena.