Erddig Castle was built as a Motte and Bailey, a type of castle which consisted of a wooden keep, built on an earthen mound or Motte, with a Bailey or defended outer ward to one side. Both mound and Bailey were encircled by a deep ditch and the earthen banks would have been topped by a timber palisade and internal walkway for defensive purposes.

The Motte and Bailey was brought to England and Wales by the Normans and represented a very successful method of establishing control over a newly conquered area.

There is a heavy concentration of these sites along the English/Welsh border demonstrating the difficulties that the Normans had in subduing the countryside.

The site at Erddig sits astride a river bluff, on one side of which ran the 8th century defensive boundary of Wat's Dyke. The bluff was cut through in two places to form the high Motte and Bailey.

The Normans probably used local labour to erect these formidable earthworks and their castle buildings.

Erddig castle was probably built at the end of the 11th century and is mentioned as "The castle of Wristleham" in a pipe roll of Henry II for the year of 1161. It seems never to have been built of stone and was abandoned as a timber castle, presumably because of its lack of continuing strategic importance so far from the border with the princes of Gwynedd.