Judges 9:51

‘ But there was a strong tower within the city, and to it fled all the men and women, and all they of the city, and shut themselves in, and made their way to the roof of the tower.'

As with many larger cities there was a fortified citadel within, and the people of the city, together with their servants and bondservants, secured themselves inside it. Then those able to help in the defence went to the roof of the tower to continue their defence by throwing from the tower on the besiegers anything available which could do them harm. They would have a pile of such things kept available at times like this.