Trilho de Offa's Dyke

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 earthwork, 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 it 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.Although the Dyke has conventionally been dated to the Early Middle Ages of Anglo-Saxon England, research in recent decades – using techniques such as radioactive carbon dating – has challenged the conventional historiography and theories about the earthwork, and show that it was started in the early fifth century, during the sub-Roman period.

Really cool trail, nice to look to one side and see England, and to the other and see Wales. Interesting that England is mostly farmland, whilst Wales is nice and wild.

Although you can barely walk about 2.5m above the surrounding land on this 12-century-old dike, you still get a nice picture of the area.
For those walking the Beacons Way, this is a nice run-up to the hills of Brecon Beacon National Park.