Fortification calls to mind images of high walls establishing clear lines between inside and outside and immobilizing enemies. However, even the most seemingly inert fortifications rely on subtle forms of mobility and more elaborate spatial relations. This article examines fortification as a technique of power in which warfare, the design of the built environment, and the organization of space are intertwined. Where research on fortification tends to emphasize the symbolic, sovereign aspirations of wall-building, the approach advanced here focuses on the spatial technologies and infrastructural projects of military architecture and engineering that remake space through martial means. The article follows the trajectory within military architecture by which linear fortifications became defense in depth and asks how transformations of ‘depth’ in contemporary warfare have come to integrate more complex, non-linear notions of space and time. By tracing the ways in which the curtain wall of Vauban’s bastion fortress transformed into the radar curtain, I argue that fortification constitutes a ‘becoming war’ in which ‘defensive’ war intensifies organized violence. As such, the concept of fortification proves indispensable for understanding the reinforced boundaries and delineated pathways cutting across the global space of contemporary warfare.