The watchman (or watchmen) is mentioned at least 35 times in the Old Testament. His role was one of great significance in keeping a city safe from attackers.
The prophet Ezekiel was appointed as a watchman over the house of Israel.
Son of man, I have appointed you a watchman for the house of Israel. Whenever you hear a word from my mouth, you must give them a warning from me. (Ezekiel 3:17 NET)
Further explanation is given in chapter 33.
But suppose the watchman sees the sword coming and does not blow the trumpet to warn the people. Then the sword comes and takes one of their lives. He is swept away for his iniquity, but I will hold the watchman accountable for that person’s death. (Ezekiel 33:6 NET)
The Psalmist reminds us that the LORD must be the true guard of a city.
If the LORD does not build a house, then those who build it work in vain. If the LORD does not guard a city, then the watchman stands guard in vain. (Psalm 127:1 NET)
This photo was made a the site of Hazor where a warrior stands in the position of a watchman over the city. The watchtower allows him to look in all directions, but especially to the north. The prophets of Israel warned of the approaching enemy from the north. See Jeremiah 1:14 and Isaiah 14:31.
The closest thing to the concept of the watchman in the New Testament is found in the description of the “leaders” among Christians mentioned in Hebrews 13:17.
Obey your leaders and submit to them, for they keep watch over your souls and will give an account for their work. Let them do this with joy and not with complaints, for this would be no advantage for you. (NET)
William Hendriksen points out that the identity of the leaders is not specified.
Those leaders who had spoken the Word of God in earlier days were no longer present. They must be remembered for their conduct and faith, says the author of Hebrews 13:7. Successive leaders have taken their place. The writer is not interested in the status of these leaders—he gives no hint whether they were elders, overseers, preachers, or teachers. Rather, he asks the reader to obey them. (New Testament Commentary: Exposition of Hebrews, 426)