Offa's Dyke, ancient entrenchment of W England and E Wales, from the Dee estuary to near the estuary of the Wye River. It was built in the 8th cent. by Offa, king of Mercia, as a barrier against the Welsh and lies mainly along the England-Wales boundary. Watt's Dyke, a similar work, roughly parallels a section of Offa's at a distance of c.2 mi (3.2 km). Parts of the dikes are well preserved.