Albert Barnes Commentary Isaiah 21:8

"And he cried as a lion: O Lord, I stand continually upon the watch-tower in the day-time, and am set in my ward whole nights;" — Isaiah 21:8 (ASV)

And he cried, A lion - Margin, ‘As a lion.’ This is the correct rendering. The particle כ (k) - ‘as,’ is often omitted (Psalms 11:1). That is, ‘I see them approach with the fierceness, rapidity, and terror of a lion .’

My lord, I stand continually upon the watch-tower - This is the speech of the watchman, and is addressed, not to Yahweh, but to him who appointed him. It is designed to show the diligence with which he had attended to the object for which he was appointed. He had been unceasing in his observation; and the result was, that now at last he saw the enemy approach like a lion, and it was certain that Babylon now must fall.

The language used here has a striking resemblance to the opening of the “Agamemnon” of Aeschylus; being the speech of the watchman, who had been very long upon his tower looking for the signal which should announce that Troy had fallen. It thus commences:

‘Forever thus! O keep me not, you gods,
Forever thus, fixed in the lonely tower
Of Atreus’ palace, from whose height I gaze
Overwatched and weary, like a night-dog, still
Fixed to my post; meanwhile the rolling year
Moves on, and I my wakeful vigils keep
By the cold star-light sheen of spangled skies.’

Symmons, quoted in the “Pictorial Bible.”

I am set in my ward - My place where one keeps watch. It does not mean that he was confined or imprisoned, but that he had kept his watch station (משׁמרת mishemeret from שׁמר shamar “to watch, to keep, to attend to”).

Whole nights - Margin, ‘Every night.’ It means that he had not left his post day or night.