A henge is a prehistoric architectural structure. In form, it is a nearly circular or oval-shape flat area over 20 metres (65 feet) in diameter that is enclosed and delimited by a boundary earthwork that usually comprises a ditch with an external bank. The earthwork permits access to the interior by one, two, or four entrances. Internal components may include portal settings, timber circles, post rings, stone circles, four-stone settings, monoliths, standing posts, pits, coves, post alignments, stone alignments, burials, central mounds, and stakeholes (English Heritage definition).