John Gill, on Lam 2:8

The Lord hath purposed to destroy the wall of the daughter of Zion,.... Either the wall of the city, as Aben Ezra; or the wall that encompassed the temple, and all the outward courts of it, as Dr. Lightfoot (s) thinks; this the Lord had determined to destroy, and according to his purposes did destroy it, or suffer it to be demolished; and so all were laid open for the enemy to enter: he hath stretched out a line; a line of destruction, to mark out how far the destruction should go, and bow much should be laid in ruins; all being as exactly done, according to the purpose and counsel of God, as if it was done by line and rule; see Isa 34:11; he hath not withdrawn his hand from destroying; till he made a full end of the city and temple, as he first designed: therefore he made the rampart and the wall to lament: the "chel" and the wall; all that space between the courts of the temple and the wall that surrounded it was called the "chel"; and so the Targum, the circumference or enclosure; and these were laid waste together, and so said to lament: according to others they were two walls, a wall the son of a wall, as Jarchi interprets it; an outward and an inward wall, one higher than another; a low wall over against a high wall; which was as a rampart or bulwark, for the strength and support of it: they languished together; or fell together, as persons in a fit faint away and full to the ground. (s) Prospect of the Temple, c. 17. p. 1089.