Fortifications are major surviving structures from the late antique world. This article demonstrates the great range of defences constructed across the East Roman Empire, beginning with a case study of the walls of Antioch based on late 18th c. engravings and revealing the scale of a major imperial 5th c. project now largely lost. The survey then reviews evidence from Asia Minor, where there is more limited evidence for new defences. On the eastern frontier, the great fortress cities are well known, but attention is drawn to fortified settlements within the frontier zone in both Roman Mesopotamia and the Balkan provinces. In the Balkan regions, however, a more elaborate response to security was the construction of a number of internal barrier walls, including the Anastasian Wall in Thrace and the newly discovered Haemus Gates. The conclusion assesses the role of fortification in the late antique world and considers the importance of providing multivocal interpretations across the frontiers of the East Roman Empire, engaging both the rich archaeological and textual sources.