Hadrian's Wall was a defensive fortification built in northern England under the orders of Roman Emperor Hadrian in 122 AD. It stretched over 120 kilometers across northern Britain to defend Roman-controlled farms and trade routes from raids by the Picts who lived north of the wall in modern-day Scotland. The wall had small forts called milecastles spaced at regular intervals along with towers called turrets. Soldiers lived in barracks within the milecastles and patrolled the wall. A ditch and road further enhanced its defenses. Hadrian's Wall marked the northernmost frontier of the Roman Empire in Britain until Roman forces withdrew in the late 4th century AD.