King Offa ruled over the Saxon Kingdom of Mercia, which spread out from the English Midlands from 757 to 796 AD. At this time the Welsh Marches were the scene of violent warfare and it is believed that it was this struggle which culminated in the building of the dyke. Research suggests that the earthwork marked, and possibly defended the western boundary of Offa's kingdom, beyond which lay the land of the Welsh Princes. During his reign, Offa introduced a penny coin on which he styled himself as King of the English, and this is celebrated in the Prestatyn logo.
The precise location in Prestatyn where King Offa's defensive dyke ended is unknown. However, a number of references place it near the remains of a farmhouse called Uffern (Hell) which once stood close to Nant Hall, on the outskirts of the town. The origins of the great earthwork are still shrouded in mystery and there are no written records from the time of construction. This means that archaeologists have to interpret the design and purpose of the dyke from the 80 miles or so that still remain. It is an earth bank up to 8 metres high, often with a ditch alongside. Unsurprisingly, the ditches have now largely filled up and become overgrown but what remains of the dyke forms the largest ancient monument in the United Kingdom.
The dyke was neglected for centuries until 1949 when it was first listed as a long distance route. The Offa's Dyke National Trail was officially opened in 1971 on behalf of the Countryside Commission.