eliminate potential cover for attacking troops. The ditches and walls channelled the attackers into carefully constructed zwinger, bailey, or similar "kill zone" areas where the attackers had no place to shelter from the fire of the defenders. Enfilade A further and more subtle change was to move from a passive model of defence to an active one. The lower walls were more vulnerable to being stormed, and the protection that the earthen banking provided against direct fire failed if the attackers could occupy the slope on the outside of the ditch and mount an attacking cannon there. Therefore, the shape was designed to make maximum use of enfilade (or flanking) fire against any attackers who should reach the base of any of the walls. The indentations in the base of each point on the star sheltered cannons. Those cannons would have a clear line of fire directly down the edge of the neighbouring points, while their point of the star was protected by fire from the base of those points. The evolution of these ideas can be seen in transitional fortifications such as Sarzana in northwest Italy. Other changes Thus forts evolved complex shapes that allowed defensive batteries of cannon to command interlocking fields of fire. Forward batteries commanded the slopes which defended walls deeper in the complex from direct fire. The defending cannon were not simply intended to deal with attempts to storm the walls, but to actively challenge attacking cannon and deny them approach close enough to the fort to engage in direct fire against the vulnerable walls. The key to the fort's defence moved to the outer edge of the ditch surrounding the fort, known as the covered way, or covert way. Defenders could move relatively safely in the cover of the ditch and could engage in active countermeasures to keep control of the glacis, the open slope that lay outside the ditch, by creating defensive earthworks to deny the enemy access to the glacis and thus to firing points that could bear directly onto the walls and by digging counter mines to intercept and disrupt attempts to mine the fort walls. Compared to medieval fortifications, forts became both lower and larger in area, providing defence in depth, with tiers of defences that an attacker needed to overcome in order to bring cannon to bear on the inner layers of defences. Firing emplacements for defending cannon were