Apse
The large stained-glass quadrifore window in the apse was made between 1328 and 1334 by Giovanni di Bonino, a glass master from Assisi. The design was probably made by Maitani. Above the altar hangs a large polychrome wooden crucifix attributed to Maitani.
Construction of the Gothic wooden choir stalls was begun in 1329 by Giovanni Ammannati together with a group of Sienese wood carvers. They stood originally in the center of the nave but were moved to the apse around 1540.
Behind the altar are a series of damaged Gothic frescoes dedicated to the life of the Virgin Mary, occupying the three walls completely. They were created around 1370 by the local artist Ugolino di Prete Ilario and a few collaborators such as Pietro di Puccino, Cola Petruccioli and Andrea di Giovanni. It took them about ten years to finish. This series of frescoes were the largest in Italy at that time. They have been restored every hundred years for several following centuries. Two scenes, the Annunciation and the Visitation, were redone by Antonio del Massaro at the end of the 15th century.
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