The historical mystery of The Castles - Bedburn - Durham England U K

An enigmatic earthwork with dry stone walls of uncertain date. ' The Castles' is a trapezoidal enclosure covering just over an acre with dry stone rubble walls averaging 16 feet thick and originally 11 feet high. There is late ridge and furrow inside the enclosure. Trenching of the site has failed to establish its function or date.
A few Bronze Age flints, including an arrowhead have been found on the site.
There is no historical documentation which provides a builder's name and a date and the structure does not appear on a map until 1820.

Who built it?
Why build such fortifications?
When was it built?

Some think it was post Roman (400-500AD) but this is just an estimate.
with nothing much to date, this massive fort remains a mystery & stimulates the imagination.

for more information
https://www.heritagegateway.org.uk/Gateway/Results_Single.aspx?uid=22237&resourceID=19191

And for those who keep watching, theres a little bonus explore at a nearby site at the end!