Fortification . a projecting portion of a rampart or fortification that forms an irregular pentagon attached at the base to the main work.

a fortified place.

anything seen as preserving or protecting some quality, condition, etc.

a bastion of solitude; a bastion of democracy.

noun

weakness , weak spot

noun

breastwork , bulwark , citadel , defense , fortification , fortress , mainstay , parapet , prop , protection , rock , stronghold , support , tower of strength , fort , pillar