The cathedral’s construction began in the early 1200s, and after almost a century, Giovanni Pisano, whose style was heavily influenced by his father, Nicola, completed the beautiful facade. Despite an impressive cathedral, Siena's ambitious ruling families decided in the 14th century to double the size of the existing structure into the New Cathedral (Duomo Nuovo). The plague stopped the plan, and today Museo dell'Opera occupies the only nave that was completed. Museo dell'Opera holds many of the cathedral's most important works of art, including Giovanni Pisano’s statues of prophets and philosophers that once adorned the facade and Duccio di Buoninsegna's Maesta. Duccio di Buoninsegna designed the huge rose window, and the gable’s glittering Venetian mosaics were added in the 19th century.