The success--however partial and hardly won--of this form of attack, led to several developments on the part of the defence. The moat was sometimes strengthened with palisading: occasionally small detached forts were constructed just outside the walls on any favourable spot. But the most generally used expedients were the brattice (_bretêche_) and the construction of large towers, projecting from the wall and flanking the long sketches of ‘curtain’ which had been found the weak point in the Norman system of fortification. The brattice was a wooden gallery fitted with apertures in its floor, and running along the top of the wall, from which it projected several feet. It was supported by beams built out from the rampart, and commanded, by means of its apertures, the ground immediately at the foot of the walls. Thus the besieger could no longer get out of the range of the missiles of the besieged, and continued exposed to them, however close he drew to the fortifications. The objection to the brattice was that, being wooden, it could be set on fire by inflammatory substances projected by the catapults of the besieger. It was therefore superseded ere long by the use of machicolation, where a projecting stone gallery replaced the woodwork. Far more important was the utilization of the flanking action of towers[67], the other great improvement made by the defence. This rendered it possible to direct a converging fire from the sides on the point selected for attack by the besieger. The towers also served to cut off a captured stretch of wall from any communication with the rest of the fortifications. By closing the iron-bound doors in the two on each side of the breach, the enemy was left isolated on the piece of wall he had won, and could not push to right or left without storming a tower. This development of the defensive again reduced the offensive to impotence. Starvation was the only weapon likely to reduce a well-defended place, and fortresses were therefore blockaded rather than attacked. The besieger, having built a line of circumvallation and an intrenched camp, sat down to wait for hunger to do its work[68]. It will be observed that by fortifying his position he gave himself the advantage of the defensive in repelling attacks of relieving armies. His other expedients, such as endeavours to fire the internal buildings of the invested place, to cut off its water supply, or to carry it by nocturnal escalade, were seldom of much avail.