This chapter explores Byzantine military architecture between 400 and 600, concentrating on the design, function and strategic significance of fortifications. It examines various defensive structures, including urban walls, military forts, civilian refuges and large-scale linear barriers. The chapter argues that fortifications were not merely passive defensive measures but played an active role in military strategy. It challenges the idea that increased fortification indicated imperial weakness, instead asserting that these defensive networks provided greater operational flexibility. Fortifications allowed armies to delay enemy advances, launch counterattacks and protect key urban centres. Additionally, the chapter highlights the evolution of fortification techniques, such as outward-projecting towers, deep ditches, reinforced gate structures and expanded urban wall circuits, demonstrating how these innovations responded to changing military threats. Ultimately, the chapter concludes that Byzantine military architecture was as much about psychological warfare as it was about physical defence. Well-designed fortifications not only deterred invasions but also reinforced imperial authority and boosted the morale of defenders, serving as both strategic and symbolic bulwarks of the empire.