Charles Ellicott Commentary 2 Samuel 20:15

"And they came and besieged him in Abel of Beth-maacah, and they cast up a mound against the city, and it stood against the rampart; and all the people that were with Joab battered the wall, to throw it down." — 2 Samuel 20:15 (ASV)

Abel of Beth-maachah. —Omit the preposition “of.” (See 2 Samuel 20:14.)

Stood in the trench. —The “trench” is the space between the wall of the city and the lower outer wall. When the besiegers had succeeded in planting the mounds for their battering engines in this space, they had already gained an important advantage.