The current, resplendent interior decoration is not Carolingian but the result of a radical restoration carried out ca. 1900 after late Baroque stucco had been stripped from the walls. It seems clear, nonetheless, that the walls were originally covered with rich marble revetment and the dome with mosaic. Hanging from the vault in the center is Barbarossa's Chandelier, a huge (4.2-meter diameter) bronze circlet commissioned by Frederick Barbarossa to celebrate Charlemagne's canonization. It was created in 1165-1184 in Aachen.

Aachen is Aix-la-Chapelle in French. Aachen Cathedral, frequently referred to as the "Imperial Cathedral" (Kaiserdom), was known as the "Royal Church of St. Mary at Aachen" during the Middle Ages. For 600 years, from 936 to 1531, the Aachen chapel (Palantine Chapel) was the church of coronation for 30 German kings and 12 queens. Dedicated to the Virgin, the chapel was nearing completion in 798, according to a letter of Alcuin. A lost inscription inside the building ascribed its construction to Odo of Metz, an individual otherwise unknown (see Schlosser, 1896). Charlemagne was buried in his chapel in 814. In 1165, through the instigation of Frederick Barbarossa, he was canonized, and his remains drew many pilgrims. Between 1355 and 1414 the eastern square apse was replaced by a double-bay apsed choir with extremely tall traceried windows and a quadripartite rib vault. The chapel was designated the cathedral of a newly constituted diocese in 1802.