Go, set a watchman - This was said to Isaiah in the vision. He represents himself as in Babylon, and as hearing God command him to set a watchman on the watch-tower who would announce what was to come to pass. All this is designed merely to bring the manner of the destruction of the city more vividly before the eye.
Related Commentaries of Isaiah 21:6
Isaiah 21:6
6 For thus hath the Lord said unto me, Go, set a watchman, let him declare what he seeth.