The Cathedral of Siena, or Il Duomo di Siena, was constructed between 1215 and 1263. Much like most other Medieval Catholic churches, the Duomo was built in the form of a Latin cross with a large bell tower and a significant dome, designed by Gian Lorenzo Bernini, which was added to the cathedral later. The colors of the cathedral’s marble facade, black and white, are symbolic colors of Siena, linked to the black and white horses of the city’s founders, Senius and Aschius. By 1215, during the very early stages of construction, there were already daily masses in the church.In 1339, there were plans to build an addition to the already extensive cathedral, which, were it finished, would have doubled the size of the structure. The addition would have included an entirely new nave and two large aisles perpendicular to the existing nave. Construction for the addition was moving along well until 1348 when it was halted by the outbreak of the Black Plague in Siena. While the Duomo is still used weekly for Sunday Mass, it is one of the most visited historical sites within the Sienese city walls and provides tourists and natives alike a look into the magnificent Medieval history of Siena. Attached to the cathedral is the Piccolomini Library where one can find illuminated choir books and frescoes painted by Bernadino di Betto.Located where the second addition of the cathedral would have been is the Museo dell’Opera. Founded in 1869, with the consent of the Ministry of Public Education, the Museo dell'Opera del Duomo is one of the oldest private museums in Italy. Many of the items, including statues, prayer books, and furnishings, came originally from the different sections of the Cathedral that have been redone over time. On the ground floor of the museum, many of the works include statues sculpted by Giovanni Pisano, Jocopo della Quercia, and Donatello. On the far end of the same floor is the beautifully complex stained glass window constructed by Duccio di Buoninsegna between 1287 and 1288, placed there in 2004. On the first floor sits the magnificent Maestà, also painted by Buoninsegna, which depicts the Madonna enthroned with the baby Christ seated on her lap surrounded by dozens of saints and angels. The rest of the museum is full of works spanning from the Medieval to the Renaissance to the modern period, including rooms dedicated specifically to paintings and tapestries.