John Gill 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
That is, the watchman cried, a lion, or that he saw a lion; not Uriah the priest, as the Septuagint; nor Habakkuk, as some Jewish writers; but Cyrus, at the head of the Persian and Median armies, compared to a lion for his fierceness, courage, and strength; see (2 Timothy 4:17) a type of Christ, the Lion of the tribe of Judah, by whom antichrist, or mystical Babylon, will be destroyed, (Revelation 5:5) . The Targum is, ``the prophet said, the voice of armies, coming with coats of mail, as a lion.'' Aben Ezra interprets it, the watchman cried as a lion, with a great voice; upon sight of the chariots and horsemen, he lifted up his voice, and roared like a lion, to express the terror he was in, and the greatness of the calamity that was coming upon the city.

I stand continually upon the watchtower in the daytime :
so that nothing could escape his notice: and I am set in my ward whole nights :
which expresses his diligence, vigilance, and constancy, in the discharge of his duty; and therefore what he said he saw might be depended on.