The Duomo (Cathedral), which dominates the center of the piazza, has a facade that is one of the earliest designed in the Renaissance manner. Although the tripartite division is conventional, the use of pilasters and of columns, standing on high dados and linked by arches, was novel for the time. The bell tower, however, has a Germanic flavor as is the layout of the Hallenkirche plan, a "triple-nave" plan where the side aisles are almost as tall as the nave; Pius, before he became pope, served many years in Germany and praised the effects of light admitted into the German hall churches in his Commentari."As you enter the middle door, the entire church with its altars and chapels is visible and is remarkable for the clarity of the light and the brilliance of the whole edifice. There are three naves, as they are called. The middle one is wider. All are the same height. This was according to the directions of Pius, who had seen the plan among the Germans in Austria" Quoted in Henk W. van Os, "Painting in a House of Glass: The Altarpieces of Pienza" Simiolus: Netherlands Quarterly for the History of Art 17.1 (1987, pp. 23-38) Works of art in the duomo include five altar paintings from the Sienese School, by Sano di Pietro, Matteo di Giovanni, Vecchietta and Giovanni di Paolo. The Baptistry, dedicated as usual to San Giovanni, is located next to the apse of the church.
