They killed the people in the fortress

46 When the leaders who lived in the tower/fortress outside of Shechem heard what had happened, they ran and hid inside the fortress, which was also a temple of their god El-Berith. 47 But someone told Abimelech that all the leaders had gathered there. 48 So he and all the men who were with him went up Zalmon Mountain which is near Shechem. Abimelech cut some branches of trees with an axe, and put them on his shoulders. Then he said to all the men who were with him, “Quickly, do what I have just done!” 49 So his men all cut branches then carried them down the mountain, following Abimelech. They went to the fortress and piled the branches against its walls. Then they kindled a fire, and the fire burned down the fortress and killed all the people who were inside. So all the people who were inside the fortress—about 1,000 men and women—died.