Hadrian's Wall in its central section does what a good border should: it picks the best ridge in the country and sits on top of it. The wall was begun in AD 122, under the emperor Hadrian, and it runs for about eighty Roman miles, roughly 117 kilometres, from the Tyne to the Solway. In the middle stretch, between Haydon Bridge and Haltwhistle, it follows a line of crags called the Whin Sill, a hard, dark volcanic rock that breaks the Northumbrian moors into a series of cliffs, so that the wall looks as if it grew out of the stone.
The Roman camp marked here is a reminder that this was an army frontier. The legions built temporary camps wherever they stopped, with turf ramparts and a ditch around them, and traces of those camps are scattered through the Northumberland countryside. Some are visible as faint rectangles in the grass, only clear when the light is low. A short distance south of the wall, the Vallum, a deep ditch with mounds on either side, ran for the whole length, and its purpose is still debated.
The best road for the wall is the B6318, the Military Road, which runs just behind it. It was built after the Jacobite rising of 1745, to help move troops across the country, and the builders used stones from the wall itself for the foundations. The road runs up and down in long, straight sweeps, with fine views north into the empty moorland of Northumberland National Park. Traffic is moderate, and in summer there can be slow caravans and cyclists, but in spring and autumn it is a delight. Fog and sudden rain come off the moor, and the wind on the crags is serious.
For a break, the fort at Housesteads, a few miles away, is the best-preserved on the wall, with a hospital and a communal latrine you can still make out. Vindolanda, the fort south of the wall, has wooden writing tablets, including a birthday party invitation, found in the waterlogged soil.
Haydon Bridge is small, and Hexham, twelve kilometres away, has fuel, supermarkets and cafés. Fill up there. Wear layers, and bring waterproofs.