the fortified perimeter wall of a city or settlement, and by extension any barrier that stands between danger and those it shelters
The word names the massive defensive wall surrounding a city or fortified town, not an interior partition or a field fence, but the perimeter structure that determined whether a settlement could resist attack. In the ancient Near East, the city wall was fundamental to survival: its height, thickness, and condition marked the difference between safety and catastrophe. This is why the prophets measure a city's strength by the state of its walls, why besieging armies aim their assault at the walls first, and why the rebuilding of Jerusalem's wall in Nehemiah is simultaneously a civic, military, and spiritual act, the restoration of the boundary between order and chaos.
The word extends naturally to any barrier that performs the same protective function. The sea standing upright at the Red Sea crossing is described with this word because it acts exactly as a city wall acts: holding lethal threat at bay on both sides. God is called a wall of fire around Jerusalem, the prophet Jeremiah is promised he will stand as a bronze wall against his enemies, and Isaiah sees salvation and praise taking the place of walls. In the Song of Songs, the image describes a woman's secure, self-possessed dignity. These figurative and divine uses are not a different concept but a genuine extension of the same idea: a firm, immovable barrier that keeps what is inside safe from what is outside.