They came, i.e. Joab and his army, which is easily understood, both from the foregoing and following verses. They cast up a bank; from whence they might either batter the wall, or shoot at those who defended it against them who should assault it. See 2 Kings 19:32 Jeremiah 32:24, Jeremiah 33:4. Otherwise, they threw down the bank of the city, which they had raised up to defend the city on the weakest side. It stood in the trench, i.e. the bank stood in or near to the trench, or wall of the city; so that the city was in great danger of being taken. Otherwise, the city stood within the trench, or wall, being defended only by a single trench, or a weak wall; the bank which was raised up there to defend it being thrown down.
Related Commentaries of 2 Samuel 20:15
2 Samuel 20:15
15 And they came and besieged him in Abel of Bethmaachah, and they cast up a bank against the city, and it stood in the trench:e and all the people that were with Joab battered the wall, to throw it down.