From the outside, this cathedral, with its rough-hewn stonework, looks more like a fortress than a church. It's the third church to stand on the site of the former 4th-century palace of the Empress Helena, mother of Constantine. It was begun in 1035 in Romanesque style, but later Gothic and baroque additions have only helped give the ecclesiastical architecture a timeless unity. The interior combines baroque decoration with Gothic vaulting and archways. On the south aisle is a magnificent tympanum depicting Christ between the Virgin and St. Peter. Die Schatzkammer (Treasury Museum) contains many important works of art, including the 10th-century St. Andrew's Altar, an unusual portable altar made of wood and covered with gold and ivory.