Northumberland, United Kingdom (UK)[22.3 miles]
One of the biggest building projects in Roman history, this wall runs for 117km across Britain from Bowness to Tynemouth. Built to keep the Scots out of England, parts are in stone, the rest made with earthen dykes and ditches. Much of it is lovely for walking, and sights along the way include forts like Housesteads and Brocolitia, with its Temple of Mithras, and a museum at Vindolandia.
Best for ages: 18+ | Free