Basilica of San Vitale
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). 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 span and height of Charlemagne's Palatine chapel was unsurpassed north of the Alps for over two hundred years.