The Aachen Cathedral is a very imposing and also a curious building: it is a mixture of different styles from different periods joined together, as you can see from the photographs above.
The most significant and interesting part of the building is the Palatine Chapel, dating from the time of Charlemagne, built at the end of the 8th century. The other parts were built over the subsequent centuries.
Aachen is worth a visit, just to see this cathedral, and can be reached quite easily from Cologne by train.