The massive keep of the Norman castles was able to resist fire and battering-ram when the besieging force came near enough to apply them. Its strength consisted in its thick walls, its height, and its massive masonry. The Edwardian castle, on the other hand, presents certain structural improvements which mark a great advance in military construction. The walls, gates, and towers are so built as to present curved surfaces to the engines of the enemy, with the result that missiles hurled against them would glance off at various angles according to the direction of the curve at the point of impact. The extent to which this development of the curve is carried in the walls of many of the Edwardian castles is quite remarkable and instructive. It shows that mere weight and bulk were no longer relied upon, but constructive skill and the judicious use of materials were guiding principles in the military architecture of the period. The following list does not include the sixteenth century blockhouses and other fortifications erected by Henry VIII, and in subsequent years. The defences on the eastern coast of England consist of an extremely interesting and important series of fortresses. In the extreme north is-- _Berwick-upon-Tweed_, a town which, from its position on the English and Scottish border, has always been a place of strategic moment, and which Queen Elizabeth spoke of as "the chief key of the realm." In the time of Edward I (1272-1307) it was encompassed by a great moat, or ditch, 80 feet wide and 40 feet deep. A crenelated wall from 15 to 22 feet high, with 19 towers at intervals, was constructed during the reign of Edward II (1307-1327). A castle had been erected at Berwick during the reign of Henry II, and together with the Edwardian wall and ditch must have formed an extremely formidable defence. The mediaeval fortifications included a large area, and in the time of Elizabeth a portion within this area was enclosed and strengthened by works of more modern character, the main features of which comprised five examples of the orillon type of bastion. The orillon was an enclosure of flattened triangular form, projecting beyond the curtain. The middle angle was obtuse, and the passage from the opening in the curtain into the bastion was somewhat restricted. It is obvious that such a bastion as this, which was introduced into England in the latter half of the sixteenth century, would give the maximum range for defensive fire, whilst affording most valuable means of protecting the flanks. The fortifications of Berwick-upon-Tweed were primarily intended for defence against the Scottish Border raiders and incursions coming overland, but they also served to protect the town against the enemy approaching by sea.