Albert Barnes Commentary Lamentations 2:8

"Jehovah hath purposed to destroy the wall of the daughter of Zion; He hath stretched out the line, he hath not withdrawn his hand from destroying; And he hath made the rampart and wall to lament; they languish together." — Lamentations 2:8 (ASV)

A line — Compare Isaiah 34:11. The destruction is systematic and thorough.