In the physical, watchmen were those set upon city walls and towers of buildings, in the day to watch over the city and the inhabitants and to warn the people of any approaching danger (cp. 2 Samuel. 18:24-27; 2 Kings. 9:17,18). In the nights they stayed awake, parading the city for the same purpose (Isaiah. 21:11,12).
In the spiritual, they were men on the height of spirituality for the purpose of alerting others of the approach of danger, and generally giving information from God (cp. Isaiah. 62:1-7; Ezekiel. 3:17-19; 33:7-9).