Ravenna was the capital of the Western Roman Empire for about 70 years until its collapse in the 5th century; then it became the capital of the Ostrogoths and later of the Byzantines in Italy.
It has 8 sites in the Unesco World Heritage list and 7 of these have amazing mosaics, all dating from the 5th/6th century.
The dome of the Arian Baptistery:
The Basilica of San Vitale:
Empress Theodora:
Queens Adrian and Franco, under the frescoed dome:
The Mausoleum of Galla Placidia:
The Neonian Baptistery:
The Basilica of San Apolinnare Nuovo:
The (mosaic-free) Mausoleum of Theodoric:
The Basilica of San Apollinare in Classe:
More pictures on Flickr: