One of the oldest cathedral buildings in Europe, it was constructed as the royal chapel of the Palace of Aachen of Emperor Charlemagne, who was buried there in 814.

First, the core of the cathedral is the Carolingian-Romanesque Palatine Chapel, which was modeled after the Basilica of San Vitale at Ravenna and is notably small in comparison to the later additions.

[7] A foundry was brought to Aachen near the end of the 8th century and was utilized to cast multiple bronze pieces, from doors and the railings, to the horse and bear statues.

[1] Due to the enormous flow of pilgrims, in 1355 a Gothic choir hall was added,[3] and a two-part Capella vitrea (glass chapel) was consecrated on the 600th anniversary of Charlemagne's death.

However, the glazing of the 14th-century choir hall, the Gothic Revival altar, a large part of the cloister, and the Holiness Chapel (Heiligtumskapelle) were irretrievably destroyed.

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 Basilica of San Vitale in Ravenna and the Little Hagia Sophia in Constantinople).

[10] 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 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.

In 1794, during the French occupation of the Rheinland, they were removed to Paris, but in 1815 up to half the pieces remaining in the Louvre were brought back to Aachen.

The original cupola mosaic was probably executed around 800 and known from Medieval sources depicted Christ as the triumphant lord of the world, surrounded by the symbols of the Four Evangelists, with the twenty-four elders from the Apocalypse of John offering their crowns to him.

Aachen Cathedral was plastered red in the time of Charlemagne, according to the most recent findings of the Rheinish Office for Monuments.

Eight was a symbol of the eighth day (Sunday as the sabbath) and therefore symbolised the Resurrection of Jesus Christ and the promise of eternal life.

The Wolfstür's imitation of the shape of the ancient Roman temple door signifies Charlemagne's claim, to have established a New Rome in Aachen with the Palatine Chapel as the distinctive monumental building.

[16] There were multiple sculptures, made of bronze, including an equestrian piece probably meant as a parallel of a statue of Marcus Aurelius in Rome.

[17] Opposite it is a bronze pine cone with 129 perforated scales, which stands 91 cm high (including its base); its date is controversial and ranges from the 3rd to the 10th century.

Its base is clearly Ottonian and includes an inscription written in dactylic hexameter, which refers to the Tigris and Euphrates rivers of Mesopotamia.

According to one view, the pine cone would originally have served as a waterspout on a fountain and would have been placed in the atrium of the Palatine chapel in Carolingian times.

[20] It is certain that the so-called Carolingian Passage entered this room on its northern wall, connecting the Aula Regia (King's Hall) in the north of the palace with the church.

The lower, barrel-vaulted room in the west probably served as Charlemagne's sepulchre after his death on 28 January 814 and his burial in the Persephone sarcophagus.

The design is arranged on the model of the Sainte-Chapelle in Paris, likewise a space for important relics and a royal palace chapel.

Clockwise from the southeast, they are: Aachen Cathedral houses a collection of medieval art objects from the late Classical, Carolingian, Ottonian and Staufian periods which are exceptional in their artistic and religious meaning.

In the western gallery on the lower floor, opposite the choir, the Throne of Charlemagne is to be found, which has been the object of new investigations in the past decades.

Between 1002 and 1014, Henry II had a pulpit erected as an ambon in the east passage, which is among the most magnificent artistic treasures of the Ottonian Renaissance.

Its inscription on the upper and lower edges clearly identifies its donor as Henry II, referring to him as REX PIVS HEINRICVS.

A golden altarpiece, the Pala d'Oro which today forms the Antependium of the high altar[21] was probably created around 1020 in Fulda.

Four round medallions with images of the Evangelists' symbols show the connection to the other twelve relief panels with depictions from the life of Jesus Christ.

They begin with the entry into Jerusalem and end with the encounter of the women with the risen Christ in front of the open grave on Easter morning.

At this time the swallow's nest organ was turned into a new, independent instrument, which now stands in the upper church, between the octagon and the choir.

After his death, on 28 January 814, he was buried in his church; the exact spot is unknown, because of the lack of documentation and the ambiguity of the physical evidence.

As early as the Middle Ages, Aachen Cathedral was admired and imitated, as in the case of Essen Minster, the Old Tower in Mettlach and in the Alsatian Abbey Church of Ottmarsheim.