One of the most prominent examples of the class is _Mam Tor_, a great hill rising to a height of 1700 feet above sea-level, and dominating Castleton and Edale, Derbyshire. Upon the summit of this eminence is a remarkable earthwork enclosing about 16 acres of land, round which the original rampart must have been nearly three-quarters of a mile in length. Natural defences of a very marked character are upon two sides of the triangular enclosure, consisting of steep slopes which descend for a considerable distance. Upon the summit of these slopes two formidable ramparts with an accompanying fosse have been constructed, thus adding still further to an almost unassailable position. The agricultural inhabitants of the district often term it "The Shivering Mountain" from the many little avalanches of shale which are dislodged from its sides. Upon the northern part the natural defences are not so apparent, as the ridge of an adjoining hill approaches at that point. An entrance to the Fort occurs there at the present time, as shown in the plan, but not in its primitive condition. The only method of entering was by means of the narrow passage shown at the S.W., defended by a fortified mound at its inner mouth, which in turn was defended by a larger mound lying to the N.W. A small spring of water still rises within the enclosure and escapes through the N.W. break. The interior has not been levelled, and a central spine of rock traverses it from north to south. Undoubtedly Mam Tor furnishes us with one of the finest examples of a fortified hilltop to be found in England. [Illustration: MAM TOR, DERBYSHIRE.] The following are a few instances of artificial defences which, although they stand upon higher ground than the surrounding land, are less dependent upon their elevated position. _Ambresbury Banks, Essex._--These banks are situated in Epping Forest, at the side of the road between Epping and London. They are of British origin, as has been definitely proved by excavations carried out by General Pitt Rivers and the Essex Field Club, thus definitely disproving the assertion previously prevailing of their supposed Roman origin. The outline approaches a square form, and this probably gave rise to the supposition. Only a few pieces of crude pottery and some flint chippings came to light during the excavations. A feature, however, was disclosed in the fosse, the lower part of which was originally of an angular section; in it a depth of silt approximating to 7 feet had accumulated. The scarp was inclined at an angle of 45°, and the counterscarp probably rose at almost the same angle; the width of the fosse was over 20 feet, and the depth above half that measurement. [Illustration: HUNSBURY, NORTHAMPTONSHIRE.]