Looming above the Roman palace of Helena (Emperor Constantine's mother), this cathedral is Germany's oldest bishop's church and still retains Roman sections. Today's edifice is a study in nearly 1700 years of church architecture with Romanesque, Gothic and baroque elements. Intriguingly, its floorplan is of a 12-petalled flower, symbolising the Virgin Mary.
To see some dazzling ecclesiastical equipment and peer into early Christian history, head upstairs to the Domschatz or around the corner to the Museum am Dom Trier.