Judith Herrin is professor emeritus in the Department of Classics at King's College London. Her books include Ravenna: Capital of Empire, Crucible of Europe; Unrivalled Influence: Women and Empire in Byzantium; Margins and Metropolis: Authority across the Byzantine Empire; Women in Purple: Rulers of Medieval Byzantium; and The Formation of Christendom (all Princeton). Phyllida Nash is an acto...