At times two bretasches were in use, one above the other; the upper projected a greater distance from the walls so as to avoid injury to the lower. The unfinished appearance of the tops of many towers can be explained by their having been covered with a bretasche in former times, although this defence was not kept in position permanently but usually built upon the approach of danger. The machicoulis and alurs of a later date were imitations in stone of the wooden bretasche. At Coucy these defences were placed about 180 feet from the ground, and the nerve displayed by the defenders working at such a giddy height excites admiration. The introduction of machicolation proper into England occurred in the latter part of the thirteenth century and became a prominent feature at that period. The faults inherent in the bretasche were the feeble resistance which it offered to missiles launched from the mangonels of the besiegers; the destruction of one part by a well-aimed stone would naturally expose the remaining defenders to archery, besides seriously weakening the rest of the structure, which depended to a great extent upon its continuity for safety. Another weakness was the perishable nature of the material, which required constant renovation and addition, and to this circumstance may be attributed the fact that examples of the true medieval bretasche are extremely rare at the present day. A fragment remains over one of the gates at Coucy, while the position of the main beam may be seen upon the outer gate of Leeds Castle. At Norham Castle a small doorway appears in the upper part of the square Keep, the conjectured use for which is that it gave access to the bretasche. In many castles of the twelfth century still remaining a line of small openings in the outer wall at the top is visible; they indicate the position of the former bretasche, and are caused by the removal of stones for the insertion of the projecting beams. Notwithstanding the advantages inherent in the Cylindrical Keep, which prompted their erection in many parts of France and other parts of the Continent, we do not find one example forming an integral part in a British Castle of the first class. [Illustration: CARNARVON CASTLE. CARNARVONSHIRE.] Cylindrical Keeps were not always of a stereotyped form, and among the comparatively few erected in England there is marked diversity in detail. Launceston, for example, really consists of a triple defence; two outermost rings of walling, one of which is a dozen feet thick and nearly 30 feet in height, effectually prevent any attempt at mining the Keep proper, which stands a few feet within the second ring. It is now only a shell, but timber flooring once divided it into three stories. The walls are nearly 50 feet in height, about 10 feet thick at the base, and stand in a ring whose diameter is nearly 20 feet. The open spaces around the Keep were formerly covered by roofing.