How did a glacis help defend a fortress?

In military science, a glacis served as a critical defensive feature by acting as a forward-sloping earthen rampart that protected the main fortress wall, or scarp, from direct enemy artillery fire.1

By sloping the ground upward toward the fortress, designers ensured that the main walls remained hidden and shielded from the line of sight of attacking batteries. Furthermore, the angle of the glacis was specifically engineered so that it could be effectively swept by cannon and harquebus fire from the parapet located behind the ditch.1

This design also incorporated a covered way—a sunken, level space situated behind the glacis—which allowed defenders to assemble safely, move troops, and prepare for sorties while remaining concealed from the view of the attackers. Together, these elements forced besieging forces to remain at a distance, significantly enhancing the survivability of the fortress against contemporary artillery.12