a wall in northern England built between 122 and 127 AD by the Roman emperor (= ruler) Hadrian, from Wallsend on the River Tyne to Bowness on the Solway Firth. It was the northern border of the Roman Empire, from which the Romans could keep back the Picts. It was a major achievement, 73 miles/120 kilometres long and 16 feet/4.9 metres high, with forts (= strong military buildings for defence) every mile along its length. Long sections of the wall still remain, and thousands of tourists visit it every year. It was made a World Heritage Site in 1987.