"I have set watchmen upon thy walls, O Jerusalem, which shall never hold their peace day nor night: ye that make mention of the lord, keep not silence" - Isaiah 62:6
During biblical times, cities were surrounded by walls for protection. The watchmen were stationed upon the walls to stand guard and keep watch for predators and enemies. When the watchmen saw an enemy approaching, they would cry out to alert the citizens. Then the people could prepare to effectively defend themselves and their city.