The latter part of the twelfth century and the earlier portion of the thirteenth was marked by the introduction of the Cylindrical Keep, forming a transition or connecting link between the Shell and the Rectangular Keeps of the previous period, and the remarkable development of castellation which occurred in the thirteenth century. The latter, however, must not be considered in the light of a sudden revolutionary change, inasmuch as many indications occur in the castles of the twelfth century which exhibit a tendency to break through the conventionalism then prevailing, and to produce works of a more complex character, suited to the progress in military methods of attack. The introduction of the Cylindrical Keep was one of these innovations; although it did not remedy the great fault inherent in Keeps generally, viz. that of impotence with regard to driving off the besiegers, yet it furnished a method which enabled the builder to effect a considerable economy in material and labour, while at the same time affording that strenuous passive resistance to assault which characterised the former styles. It is probable that King Henry II. was chiefly responsible for the introduction of the Cylindrical Keep, as by reason of his French birth he was acquainted with a number of foreign castles having citadels built upon this plan. These Cylindrical Keeps were likewise known as Donjons and Juliets, and attained to a degree of perfection upon the Continent which was never reached in the British Isles. The example at Coucy is probably the finest abroad. The advantages which may be claimed for the Cylindrical Keep, apart from its lessened cost of construction, are the increased solidity, and the great difficulty in breaching it, or bringing it down by a mine. By vaulting each floor the resistance of the structure was increased; by enclosing the upper part in a similar manner also, the danger of fire from incendiary missiles launched upon the roof was practically nullified. A disadvantage, however, lay in the fact that the besieged could not concentrate a discharge of missiles against assailants at one part of the base without exposing themselves to the enemy's archery. This was to a great extent rectified by the bretasche, which, though in use previously, became established as a regular defence at this period. These were timber galleries encircling the outer part of the tower at its summit, supported in position by strong beams of wood inserted in holes made for the purpose, and strengthened by struts resting upon corbels. Upon this foundation a wooden gallery was built, covered in by a sloping roof resting against the walls, and generally enclosing the summit of the wall. In suitable places the gallery was loopholed for archers and cross-bowmen, while through openings in the floor stones and other missiles could be dropped upon assailants at the foot of the Keep. It could be entered from the battlements behind, where stores of ammunition were placed.