The cathedral was built between 1025 and 1061 at the initiative of Emperor Conrad II, who is buried in the crypt alongside seven other emperors and kings. It is an immense triple-nave basilica crowned by four towers, and was the largest Romanesque structure in existence until the building of Cluny. A UNESCO World Heritage Site, it is considered by many to be the finest Romanesque cathedral in the world.
The imperial crypt is the most affecting part of the visit. Eight Holy Roman Emperors and kings lie here — Conrad II, Henry III, Henry IV, Henry V, Philip of Swabia, Rudolf of Habsburg, Adolf of Nassau, and Albert I — in a low, barrel-vaulted chamber with polished stone sarcophagi. There is nothing ostentatious about it; the restraint makes it more powerful. The cathedral itself is built in warm sandstone that glows orange-red in late afternoon light, and the sheer scale of it becomes clear only when you walk around the outside and see how far it extends along the Rhine plain.
The interior was heavily restored in the 19th century, which means it is tidier and more complete than most Romanesque churches of its age, but also less raw. The apses at the east end, with their blind arcading, are the most purely Romanesque parts. Visit on a weekday morning if you can — weekend afternoons bring school groups and tour coaches, and the atmosphere is better when it is quiet.