Hadrian's Wall was built by the Roman Empire to protect their territory in England from the Pictish tribes of Scotland. It was built relatively quickly from 122 AD, with associated castles and forts, and stretches for 73 miles / 117 km from the Tyne estuary on the east coast to Solway Firth on the west coast. The best of it, both for its preserved structure and scenic upland location, is the central 20-or-so miles across Northumberland between Hexham and Haltwhistle. Hadrian's Wall is inscribed on the UNESCO World Heritage List.
Established emperors can skip this bit, but newbies start here: in order to protect your homeland, power base and own sweet self, you have to subdue the surrounding territories. Exploit them for the benefit of self and homeland (for dammit, you are the homeland) as ruthlessly as you dare without provoking rebellion. Recruit their young men into your army with steady pay, fine uniforms and tales of glory; send them to occupy a different territory where they've no local loyalty, will brutally do your bidding, and are politically expendable. Transition the military territory into a civic province dotted with your noble statue. But in order to protect it, you have to subdue the surrounding territories.
The entire route of the wall has been turned into a long distance footpath. For about 20 miles either side of the central section, this follows a well-defined earthwork descending west towards Brampton and east towards Prudhoe.