Who Roman Emperor built a massive wall across northern Britain in AD?

Hadrian built the wall across the width of the northern part of Britain - about 73 miles - with a series of military forts, mainly to keep out the warring tribes to the north (in what is now Scotland) and to mark the boundary of the Roman Empire. It was ordered to be built by the Roman Emperor Hadrian, when he visited Britain in AD 122. Much of it still remains to this day, and it is a popular tourist attraction. It's called Hadrian's Wall.