The cathedral's centerpiece is its organ, which has an impressive 7,896 pipes and 110 registers, allowing it the dynamic range to produce rousing renditions of Beethoven's masterpieces, the chirping of birds, the humming of human voices, or the rippling of water. Johann Wuff constructed the organ, which has an oval stained glass in the middle and is flanked by cherubs and trumpet-wielding angels, from 1755 to 1780. At the time, it was the largest instrument of its kind in Europe. The imposing three-nave vaulted basilica started life as a wooden structure in the 12th century. Since its expansion in 1224, it has had its share of fires and plunders through the ages by Prussians, Teutonic Knights, the Swedish invasion, and World War II. Time your visit with the free organ concerts. During the 20-minute concerts, the main entrance is closed, but you can gain access via the side entrance.