On my way from Koln to Amsterdam yesterday, I made a brief (about 90 minute) stop-off in Aachen, situated right where Germany borders both Belgium and the Netherlands. The reason was to see the Aachen Dom, another highly impressive cathedral that in this case holds the remains of Charlemagne.
Built in about 800, Charlemagne built it in part to cement Aachen as the capital of his empire, which is arguably the first European empire (although it didn't include Spain, Britain, Scandanavia, or Eastern Europe). The throne in the cathedral was used to crown about 30 German emperors, and has been a pilgramage site for some time.
As with most of these cathedrals, this one also claims to have a holy relic, in this case Christ's loincloth. It has some other lesser relics, and they are displayed once every 7 years (next in 2014).