These watchmen are on the walls of Jerusalem and are engaged in worship, prayer and praise. Since no men are there, these watchmen are celestial beings. They are continually praising God and are engaged in divine services. They are the people who know God's will as revealed in the Scriptures, and who are urged by the Lord continually to bring in faith before God, the divine plan. Thus God's servants cooperate with Him in prayer.
Prayer changes things.
Prayer changes people. Prayer brings victory. It behooves every true servant of the Lord to study the Scriptures faithfully in order that he might know God's will, as revealed in the Word, and in order that he might pray intelligently. In other words, the Lord wants a volume of believing prayer, regarding the future of Jerusalem and the Jewish people, ascending to His throne day and night, for prayer changes things. This highway is to be made for the coming of Messiah.