Wat's Dyke is an ancient defensive earthwork between Wales and England. The dyke is earlier than Offa's Dyke, but runs along similar lines, just inside the line of the later dyke. It now looks insignificant, but originally it was a considerable construction.
The dyke consists (or consisted) of a ditch and a bank. The ditch is on the west of the bank. This suggests it was meant to keep the Welsh tribes out of the areas to the east which were occupied mainly by Germanic tribes.
Excavations in 2006 suggested the time of building as 792–852 AD. In the 820s, the Mercian king Coenwulf was fighting against a resurgent Welsh threat. That may have been the reason for the dyke. However, all previous datings suggested its being built much earlier, in the post-Roman era.
The present boundary between Wales and England runs close to the two dykes, and the area is know as the Welsh Marches.