Much overgrown walling,very low down the site.Possibly part of the original structure(Welsh).The folly walling atop the site,is not connected to this.
see http://www.ecastles.co.uk/walesgr.html
see http://cy.wikipedia.org/wiki/Castell_Deudraeth
Note:To all Welsh speakers;the previous reference,in Welsh,I cannot read,but,it is included here(I hope it's nothing to do with the hotel of the same name,also in the grounds at Portmeirion),because I recognise all the other entries on the wikipedia page,as being Welsh princes castles of the same period.