One of the oldest and the most important churches in Europe - Aachen Cathedral. The oldest part of the church, Palatine Chapel, was built by Emperor Charlemagne and was consecrated in 805.
Photo: Cathedral internals
Do not forget to visit upper floor with Throne of Charlemagne. You can take part in a guided tour. It costs 4e and lasts 45 minutes. Unfortunately it is forbidden to visit upper floor alone without guide.
Next to the church you can find museum/treasury with famous Bust of Charlemagne reliquary.