Episode 4 considers the ways in which armies have used or modified the terrain of the battlefield to their advantage for both defense and attack. From Hadrian’s Wall and the Great Wall of China, to the mountain fortress of Masada and the great ramp the Romans built to conquer it, to the rooftops and alleyways of Baghdad, battlefield engineers have worked creatively for thousands of years to make their landscapes work for them. Massive Crusader castles like Krak des Chevaliers, with its multiple walls and concentric layers of defenses, provided “defense in depth,” as did later features like bastions, star forts, moats, and earth-backed walls. But offense too benefited from battlefield engineering, as soldiers tunneled under enemy lines, dug complicated systems of trenches, and used barbed wire to funnel enemies into deadly killing zones. Even in today’s conflicts, the shovel remains a staple of battlefield gear, and the manipulation of the conflict zone can be the difference between victory and deadly defeat.