Periwinkle Hill is possibly a Norman earthwork motte and bailey fortress, belonging to the chief manor of Reed. It could also be a small moated mound with squarish corners and an enclosure, built in the Anarchy in the reign of King Stephen. Pottery has been found but the classification of the earthwork as a oval motte and bailey can be neither confirmed nor denied by ground inspection. The site is now part of a corn field and ploughing is making the site and its layout very hard to observe. 3 miles west is Therfield Castle and 4 miles south-east is Anstey Castle.