Offa’s Dyke (Welsh: Clawdd Offa) is a large linear earthwork that roughly follows the current border between England and Wales. The structure is named after Offa, the Anglo-Saxon king of Mercia from AD 757 until 796, who is traditionally believed to have ordered its construction. Although its precise original purpose is debated, it delineated the border between Anglian Mercia and the Welsh kingdom of Powys.

The dyke, which was up to 65 feet (20 m) wide (including its flanking ditch) and 8 feet (2.4 m) high, traversed low ground, hills and rivers. Today the earthwork is protected as a scheduled monument. Some of its route is followed by the Offa’s Dyke Path; a 176-mile (283 km) long-distance footpath that runs between Liverpool Bay in the north and the Severn Estuary in the south.

Participants taking up the challenge  often start from south to north, starting by the Severn Estuary at Sedbury, near Chepstow and finishing at Prestatyn on the north coast, the walk will take an average walker roughly 12 days to complete. Following a man-made border and ancient monument, rather than natural features, the dyke path crosses a variety of landscapes. The route crosses the Black Mountains, the Shropshire Hills, including the many ups and downs of the ‘Switchback’, the Eglwyseg moors north of Llangollen and the Clwydian Range.