'Watchman' in the Bible

Now David was sitting between the two gates: and the watchman went up to the roof of the gate unto the wall, and lifted up his eyes, and looked, and, behold, a man running alone.

And the watchman cried, and told the king. And the king said, If he be alone, there is tidings in his mouth. And he came apace, and drew near.

And the watchman saw another man running; and the watchman called unto the porter, and said, Behold, another man running alone. And the king said, He also bringeth tidings.

And the watchman said, I think the running of the foremost is like the running of Ahimaaz the son of Zadok. And the king said, He is a good man, and cometh with good tidings.