These are probably Gerizim and Ebal, which overlook Shechem from the southwest and northwest respectively (Judges 9:7; see also Deuteronomy 11:29; Deuteronomy 27:12–13; Joshua 8:23). They would have provided excellent vantage points from which to see anyone approaching the narrow Shechem pass below.
25 And the leaders of Shechem put men in ambush against him on the mountaintops, and they robbed all who passed by them along that way. And it was told to Abimelech.