Radak citing Targum Yonasan: It is a mound of earth. They fill the cavities around the city [wall] and make a mound to [ascend it and] reach the wall.

Rashi citing Targum Yonasan: Camps surrounded [the city].

Rashi: Some say that it is like "Chel v'Chomah" (Eichah 2:8, a wall within a wall). They toppled the outer wall, and were standing by the inner wall. Radak - the city was ready to be conquered. They Yonasan: It is a mound of earth. They already filled the cavities and only needed to topple the wall.

Radak citing Targum Yonasan: They stood in the cavities, i.e. after they filled them, and stood at the bottom of the wall.

Malbim: The Solelah did not help, because the [people of] city stood by the Chel in front of the wall. Therefore, they wanted to topple the wall.

Radak: They used to a Kli to shatter rocks, to topple the wall.

Radak citing Targum Yonasan: They were thinking how to topple the wall.