raising the bank on either side. The enemy, making for this point, would miss the real entrance, and run the risk of losing his life in a _cul-de-sac_ purposely constructed within the rampart. [Illustration: Blackbury Castle (From Mr Hadrian Allcroft’s _Earthwork of England_ by kind permission of Messrs Macmillan & Co.)] In these fortresses, the defences of which were due to instinctive skill in the face of constant danger, it is impossible not to recognise how many of the most scientific features of medieval fortification were anticipated. The concentric plan of Caerphilly castle (270), with its easy provisions of egress, and its difficulty of access; the spurs which guard the approaches to Beaumaris (277) and Conway castles; the barbicans which form so prominent a defence of Alnwick castle (243), and the gateways of York have prototypes, of which their engineers were probably unconscious, in the huge earthworks of prehistoric Britain. Through a long interval, in which military art pursued a very gradual evolution, the wheel came full circle. The devices of the earthwork builders were translated into stone with far greater economy of labour. Although the most conspicuous examples of early earthwork are found in hill fortresses, camps were not confined to hilly sites, nor were their defences always composed of earthwork. There are districts where the hard nature of the soil forbade the construction of earthen banks and fosses; and consequently there are many camps which are surrounded by walls of rough uncemented stone, originally kept in place by facings and bondings of larger and smoother stones.[4] These camps are usually not large. They occur very commonly in the north of England: a good example is that known as the Castles, in the valley of the Bedburn, seven or eight miles west of Bishop Auckland. The enclosure, situated on a boggy slope, is surrounded by shapeless masses of _débris_, the ruins of the dry-built ring wall, which has lost its facing and so has fallen to pieces. Stone was also used in the ramparts of some of the camps on the rocky hills of Somerset, as in the camps on either side of the Avon at Clifton. The great fortress of Worlebury, above Weston-super-Mare, was surrounded by an immense wall of uncemented stone, brought to great thickness by building several walls, each with its own set of facing stones, up against each other. On its eastern front, separated from the main rampart by a deep ditch, cut in the solid rock, was another wall of stone; and this again was protected by a series of outer earthworks (9). Dolebury, at the western extremity of the Mendips, is surrounded by a double wall of loose limestone.[5] In these cases, as in the Welsh strongholds of Penmaenmawr and Tre’r Ceiri, geological conditions made the earthen bank an impossibility, and the stone of the neighbourhood took its place. [Illustration: Worlebury