I have visited this site in the same day as Augustusburg in Bruhl. These two sites plus Cologne cathedral are close to each other so it is a good hit for WH hunters :)

The cathedral itself is "different" among others on the first sight. It is visible that is a mixture of different architectional styles - Gothical and Romanic (or Byzantine). Later as i read it is because of huge volume of pilgrimes.

Interior is very interesting - the Byzantine part is build in Octagon shape (8 sides) and has beautiful overhead decoration. Gothic part has nice long windows so you can feel the atmosphere of the history there.

Unfortunately i came quite late the guided tour had already started so i was not able to see the Charlemagne's throne and other parts of cathedral.

Pic: top of the octagon