Iran and surrounding territories were moulded into a strong state in the Sasanian period, i.e. an institutionally developed political entity, managed from an imperial centre. Leaving aside the testimony of clay sealings for different branches of provincial administration and of campaigns for military capability, incontrovertible evidence for the existence of a governmental system with effective outreach and downreach is provided by military construction projects and the monetary system. As regards military infrastructure, attention is concentrated on works undertaken on both sides of the Caspian: two formidable lines of defence were created at narrow points in the passage between the Caucasus and west coast, to be dated probably to the late fourth and early fifth centuries, the front line being upgraded probably toward the end of the reign of Kawad I (488–96, 499–531); to the east where the Gurgan plain offered easy access to the southern shore of the Caspian and passes across the Elburz mountains, a grandiose scheme of linear defence was introduced, probably in the middle decades of the fifth century, consisting of a 200 km long wall guarded by more than thirty-six forts and fronted by a wide, water-filled ditch, several ready-made fortified camps for field forces to the rear and an associated wall across the eastern end of the southern coastal plain. The financial foundations of the state can be examined through study of the coinage, which has been subject to thorough-going analysis in recent publications. Despite a proliferation of mints, there was an impressive uniformity of types in the main silver currency, changes being introduced in a single coordinated operation. The high purity (averaging over 95%) of the silver drachm was maintained from the fourth century to the early 620s, even at times of serious financial strain. Preliminary studies of variations in overall mint output and by region cast additional light on economic and organisational history, while policy initiatives and associated propaganda can be glimpsed thanks to the survival of special issues (in gold as well as silver).