Albert Barnes 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)

Cast up a bank - See the marginal references. The throwing up of mounds against the walls of besieged places by the besiegers is well illustrated in the Assyrian sculptures.

The trench - The “pomoerium,” or fortified space outside the wall. When the mound was built in the pomoerium, the battering engines were able to approach close to the wall to make a breach.