Adam Clarke on 2 Samuel 20:15

They cast up a bank against the city - The word סללה solelah, which we render bank, means, most probably, a battering engine of some kind, or a tower overlooking the walls, on which archers and slingers could stand and annoy the inhabitants, while others of the besiegers could proceed to sap the walls. That it cannot be a bank that stood in the trench, is evident from the circumstance thus expressed.