And he sent messengers unto Abimelech privily, saying, Behold, Gaal the son of Ebed and his brethren be come to Shechem; and, behold, they fortify the city against thee. They fortify the city against thee - Under pretense of repairing the walls and towers, they were actually putting the place in a state of defense, intending to seize on the government as soon as they should find Abimelech coming against them. Fortifying the city may mean seducing the inhabitants from their loyalty to Abimelech.
Related Commentaries of Judges 9:31
Judges 9:31
31 And he sent messengers unto Abimelech privily,j saying, Behold, Gaal the son of Ebed and his brethren be come to Shechem; and, behold, they fortify the city against thee.