https://www.theguardian.com/science/2016/oct/03/skipsea-castle-yorkshire-built-on-iron-age-mound?CMP=share_btn_tw
as per usual a good article from the Guardian and not the Daily Mail this time! :)
I wasn't sure what the basis for dating the mound was, though.
Given it was done by coring, extraction of organics from the base layer and radio-carbon dating them would be my guess.