The Roman emperor who built a defensive wall across northern Britain was Hadrian.
Construction of Hadrian’s Wall began in AD 122 during the reign of Emperor Hadrian, after unrest in Roman Britain and a broader imperial shift away from constant expansion. The wall crossed northern England from Wallsend on the River Tyne to Bowness-on-Solway, forming an approximately 73-mile frontier installation.
The project was carried out by Roman soldiers, including troops from three legions: Legio II Augusta, Legio VI Victrix, and Legio XX Valeria Victrix. The frontier included forts, milecastles, turrets, gates, ditches, and a major earthwork known as the Vallum. It was more than a solid barrier: controlled gates helped regulate movement, trade, taxation, and military response.
The wall is sometimes incorrectly described as the border between England and Scotland; it lies entirely within England. Another common confusion involves the Antonine Wall, built farther north in what is now central Scotland under Antoninus Pius around AD 142. That later frontier briefly replaced Hadrian’s Wall but was eventually abandoned, after which Roman control returned to Hadrian’s line.