Which Roman Emperor ordered the construction of a wall in northern England to keep out the Picts?

Hadrian's Wall is one of the most iconic Roman structures in Britain, and its construction tells us a lot about the Roman Empire's strategy. The Romans had conquered much of Britain by the first century AD, but they struggled to control the northernmost parts, particularly the lands inhabited by the Picts, who were known for their fierce resistance. Emperor Hadrian visited Britain around 122 AD and, recognizing the difficulty of conquering the Picts outright, decided on a different approach: containment. He ordered the construction of a wall stretching across northern England, from the River Tyne to the Solway Firth. This wall, known as Hadrian's Wall, served not only as a physical barrier but also as a symbol of Roman power and a controlled border point. It allowed the Romans to regulate trade, control movement, and defend against raids from the north. While later emperors like Antoninus Pius did build other, more northerly walls (like the Antonine Wall in Scotland), Hadrian's Wall was the first major defensive structure and is the one most famously associated with Roman Britain.