Hadrian's Wall (near Hexham, Northumberland): A World Heritage Site, this wall -- now in ruins -- was ordered built by Hadrian, the Roman emperor, in A.D. 122 to hold back barbarian invasions from the north. Marking the far northern border of the Roman Empire, the wall stretched 118km from Wallsend, or Wall's End, north of Newcastle upon Tyne in the east to Bowness-on-Solway beyond Carlisle in the west. A milecastle (small fort) was added at every mile along the wall. A highlight is Vindolanda, the last of eight successive Roman forts built on a site adjacent to the wall.