Therfield Castle is a 12th century earthwork motte and bailey fortress, built in the period of the Anarchy. The small motte has a shallow depression in the top and is partly enclosed by a ditch. Excavated in 1958, the motte was found to be absent of any structures but the rectangular bailey was built with a timber-revetted rampart. In 1960, most of the earthworks in the bailey were levelled, leaving only wet ditches and low ramparts in the field boundaries, to give its layout. After the Anarchy, the castle may have been slighted before its construction was complete. 3 miles east is Periwinkle Hill and 10 miles south is Walkern Castle.