The Treasury of Aachen Cathedral — the Domschatzkammer — holds one of the most important collections of medieval sacred art north of the Alps. The highlights are Charlemagne's personal relics: his hunting horn (a carved Byzantine ivory), the Cross of Lothair (a 10th-century processional cross encrusted with gems and cameos), a gilded bronze equestrian statue of the emperor, and the Shrine of Charlemagne, a gold reliquary in the shape of a miniature cathedral that contains the emperor's skull and arm bones.
Less famous but equally fine are the textiles: Carolingian silk robes, Byzantine cloths, and a 10th-century silk vestment from Constantinople still in remarkable condition. The collection fills a modern space beneath the Dom that manages to present these objects without overwhelming them. Allow at least an hour; the audio guide is unusually good.