The architect of Charlemagne's church was a man named Odo of Metz. He was inspired by Byzantine architecture and modeled this church after a church in Ravenna and the Hagia Sophia and Little Hagia Sophia in Istanbul. This church is octagonal in design with an octagonal dome-cupola. There have been additions over the centuries of towers, a choir and additional chapels. Below is an exterior photo of the current cathedral. You can see the octagonal structure of the original palatine chapel in the center.
The photo below shows the entrance to Aachen Cathedral. The doors in the center at street level are knows as the Wolf's Doors. They were cast of bronze in Aachen in about 800 AD. The next two photos are closeups of these doors, including the Lion's Head handles. The doors weigh about 4 tons.
This is the main floor of the chapel looking towards the choir, which was a later addition.
The chandelier seen in the above photo and in the photo below was given to the cathedral by the Emperor Frederick Barbarossa and his wife, Beatrice, around 1165 AD. It still utilizes candles and is lit only on special occasions. It takes most of a day for a crew to put in new candles.
Charlemagne's throne is located in the gallery above the main floor. It is marble; some of the stones under the throne on the platform came from Jerusalem. It looks plenty uncomfortable but I guess if you covered it with furs, silks, pillows, etc. I suppose it would be okay. It is shown in the two photos below. Between 936 and 1531 AD 30 German kings and 12 German queens were crowned here using this throne.
On his death in 814 AD Charlemagne was buried in the chapel. In 1165 AD Frederick Barbarossa had Charlemagne's remains put in a gold box, which is on the gallery floor near the throne. Carlemagne was also canonized in 1165 AD. This box is shown below.
The photo below shows the bone box from the end. Frederick Barbarossa made a political statement with this end. Charlemagne is shown as the middle figure with Christ above him. There is a church official on either side of Charlemagne with their heads bowed.
Below is the mosaic on the dome of the cupola, seen from the gallery near the throne.
We had a little friend with us who managed to get an Aachener Dom cookie, available only in Aachen. She managed to keep it away from the cow, and sat in a throne on the square.