The Dom (officially the Stiftskirche of the Marian collegiate chapter) is famously unfinished. Construction began around 1230 on the foundations of an earlier Carolingian and Romanesque church, and by the time the money and ambition ran out in the 16th century the building had two mismatched west towers, a finished Gothic choir, and a nave in three different styles. The result is a hybrid that reads like an architectural cross-section of the late Middle Ages.
The interior is unexpectedly harmonious, with 14th-century frescoes, an unusually expressive late-Gothic Pietà from around 1380, and the dramatic light from the south rose window. Free to enter; small fee for the treasury.