Adam Clarke on Nehemiah 4:20

Ye hear the sound of the trumpet - As the walls were very extensive, and the workmen consequently much scattered, their enemies might easily attack and destroy them successively, he therefore ordered them all to work as near to each other as they could; and himself, who was everywhere surveying the work, kept a trumpeter always with him, who was to sound when the enemy approached; and all were instantly to run to the place where they heard the sound.