What does Isaiah 21:7 mean?
The Lord has called for a watchman to be stationed on the walls to report what he sees coming (Isaiah 21:6). In a sense, Isaiah himself may be that person standing on the walls of Jerusalem waiting for word to arrive that Babylon has fallen. Or he is calling for the watchmen in Babylon to keep a lookout for the enemy threat to arrive.Now he instructs the watchman to be fully on alert when he sees riders and horsemen approaching the city in pairs. Whether those riders are on donkeys, horses, or camels, he should listen very closely to what they report.
The prophet may have been describing the approach of the enemy warriors toward the walls of Babylon as if he were seeing it in his vision. It is difficult to know exactly since the entire oracle in chapter 21 is almost delivered as if it were a dream. The precise details matter less than the overall message of dread and suffering at the judgment to come.