The Roman Empire, and its eastern and western successor states, controlled the majority of Europe’s population for approximately half a millennium (first century BC to fifth century AD), holding dominant power status from the second century BC to the seventh century AD, longer than any other state in the western world in history, and it was also the only empire ever to rule over the entire Mediterranean. Its ability to integrate ethnic groups and its well-organised military apparatus were instrumental to this success. From the third century onwards, however, the balance increasingly shifted; the physical dimensions of fortresses and unit sizes tended to decrease markedly in the Roman world, and the tradition of constructing marching camps and training facilities seems to have been abandoned. By contrast, the Sasanian Empire increasingly became the motor of innovation. Already in the third century it matched Rome’s abilities to launch offensive operations, conduct siege warfare and produce military hardware and armour. Jointly with the Iberians and Albanians, the empire also made skilful use of natural barriers to protect its frontiers, notably by blocking the few viable routes across the Caucasus. By the fifth/sixth century, it pioneered heavily fortified, large, rectangular campaign bases, of much greater size than any military compounds in the late Roman world. These military tent cities, filled with rectangular enclosures in neat rows, are suggestive of a strong and well-disciplined army. Like these campaign bases, the contemporary c. 200km-long Gorgan Wall, protected by a string of barracks forts and of distinctly independent design, is not copied from prototypes elsewhere. The evidence emerging from recent joint projects between the Iranian Cultural Heritage, Handcraft and Tourism Organisation and the Universities of Edinburgh, Tbilisi and Durham suggests that in late antiquity the Sasanian army had gone into the lead in terms of organisational abilities, innovation and effective use of its resources.