Above the arches of the gallery, an octagonal drum with window openings rises, on top of which is the cupola. This area was connected with the palace by a passage. The high altar and Imperial throne are located on the upper circuit of the Palatine chapel in an octagonal side room, covered by a barrel vault lying on an angle. The two floors are separated from each other by an expansive cornice. The arched openings of the lower story are only about half as high as those of the HochmГјnster, as a result of which the lower story looks stocky and bulky. This upper story was known as the HochmГјnster (high church).

Around this inner octagon is a sixteen sided circuit of low groin vaults, supporting a high gallery above. The inner octagon, with a diameter of 14.46 metres (47.4В ft), is made up of strong piers, on which an octagonal cloister vault lies, covering the central room. The Palatine chapel consisted of a high octagonal room with a two-story circuit below. The span and height of Charlemagne's Palatine chapel was unsurpassed north of the Alps for over two hundred years. The architect was Odo of Metz, and the original design was of a domed octagonal inner room enveloped by a 16 sided outer wall. the Basilica of San Vitale in Ravenna and the Little Hagia Sophia in Constantinople). The octagon in the centre of the cathedral was erected as the chapel of the Palace of Aachen between 796 and 805 on the model of other contemporary Byzantine buildings (e.g. The Barbarossa chandelier under the dome of the Octagon Finally, there are portions that show Ottonian style, such as the area around the throne. Secondly, the choir was constructed in the Gothic style. First, the core of the cathedral is the Carolingian- Romanesque Palatine Chapel, which was modeled after San Vitale at Ravenna and is notably small in comparison to the later additions. The cathedral uses two distinct architectural styles, with small portions of a third. In 1978, Aachen Cathedral was one of the first 12 items to be listed on the UNESCO list of World Heritage sites. Reconstruction and restoration took place intermittently over more than 30 years, and cost an estimated в‚¬40 Million. However, the glazing of the 14th-century choir hall, the Neo-Gothic altar, a large part of the cloister, and the Holiness Chapel ( Heiligtumskapelle) were irretrievably destroyed. Many of the cathedral's artistic objects had been removed to secure storage during the war, and some which could not be moved were protected within the church itself. Дuring World War II, Aachen, including its famed cathedral, was heavily damaged by Allied bombing attacks and artillery fire, but the cathedral's basic structure survived.

It was restored again in 1881, when the Baroque stucco was removed. A cupola, several other chapels, and a steeple were also constructed at later dates. Due to the enormous flow of pilgrims, in 1355 a Gothic choir hall was added, and a two-part Capella vitrea (glass chapel) which was consecrated on the 600th anniversary of Charlemagne's death.

Floorplan of Charlemagne's Palatine ChapelДЇollowing Charlemagne's canonization by Frederick Barbarossa in 1165, the chapel became a draw for pilgrims.

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