Hadrian's Wall was built in 122 AD by the Roman Emperor Hadrian across northern England as a defensive fortification. It was constructed to protect Roman Britain from raids by northern tribes like the Picts after the country became prosperous with farms and trade. At over 120 kilometers long, it was the most heavily fortified border in the Roman Empire and featured small forts called milecastles and towers called turrets. It remained occupied by Roman troops until the fall of the Roman Empire.