Basilica of San Vitale
There is a sixteen-sided ambulatory with a gallery overhead encircling the central octagonal dome. The plan and decoration owe much to the sixth-century Basilica of San Vitale, Ravenna. Indeed, Charlemagne visited Ravenna three times, the first in 787. In that year he wrote to Pope Hadrian I and requested "mosaic, marbles, and other materials from floors and walls" in Rome and Ravenna, for his palace.