John Calvin 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. “Having listened diligently with careful attention,” he finally observes a lion. This is understood to mean Darius, who conquered and pillaged Babylon, as we learn from Daniel (Daniel 5:28, 31).

I stand continually. When the watchman says that he is continually on his watchtower by day and by night, this tends to confirm the prediction, as if he had said that nothing can be more certain than this vision; for those whom God has appointed to keep watch are neither drowsy nor dim-sighted. Meanwhile, by this example, he exhorts and stimulates believers to the same kind of attention, so that by the help of the lamp of the word, they may obtain a distant view of the power of God.