Offa's Dyke, series of huge earthworks created between ad 784 and 786 by the Anglo-Saxon king Offa to mark the boundary between England and Wales. The dyke is the largest archaeological monument in Britain, running some 240 km (150 mi) from the mouth of the River Wye in the south to Basingwerk, on the banks of the River Dee, in the north. It consists of a large earth rampart rising more than 7 m (25 ft) on the eastern side and a ditch approximately 1.8 m (6 ft) deep on the western side.

Offa was the king of Mercia and the dominant leader of Anglo-Saxon England. Apart from the dyke itself, there is other evidence to suggest that relations between England and Wales were hostile. This would explain why Offa built such a massive barrier to protect his territory. Although there are gaps in its length today, it is thought that it originally ran continuously along the whole north-south frontier.

Offa's Dyke stands as a monument not only to the power and resources of Anglo-Saxon England and of Offa himself, but also of the success of the Welsh in their struggles with the Anglo-Saxons. In this context it is also worth noting the existence of a (more modest) Welsh monument at Valle Crucis near Llangollen, erected in memory of Elise, the chief Welsh opponent of Offa.