Bastion Fort

Passive ring-shaped (enceinte) fortifications of the Medieval era proved vulnerable to damage or destruction by cannon fire, when it could be directed from outside against a perpendicular masonry wall. When the newly-effective maneuverable siege cannon came into military strategy in the fifteenth century, the response from military engineers was to arrange for the walls to be embedded into ditches fronted by earthen slopes (glacis) so that they could not be attacked by destructive direct fire and to have the walls topped by earthen banks that absorbed and largely dissipated the energy of plunging fire. 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.

Source: en.wikipedia.org