 As was the case under Justinian 's predecessors , the Empire 's economic health rested primarily on agriculture . In addition , long @-@ distance trade flourished , reaching as far north as Cornwall where tin was exchanged for Roman wheat . Within the Empire , convoys sailing from Alexandria provided Constantinople with wheat and grains . Justinian made the traffic more efficient by building a large granary on the island of Tenedos for storage and further transport to Constantinople . Justinian also tried to find new routes for the eastern trade , which was suffering badly from the wars with the Persians . 
