Isaiah 21 contains oracles concerning Babylon, Edom, and Arabia. The watchman is to describe what he sees. From his watchtower where he stood day and night he was able to announce the fall of Babylon.
For thus the Lord said to me: “Go, set a watchman; let him announce what he sees. When he sees riders, horsemen in pairs, riders on donkeys, riders on camels, let him listen diligently, very diligently.” Then he who saw cried out: “Upon a watchtower I stand, O Lord, continually by day, and at my post I am stationed whole nights. (Isa 21:6-8 ESV)
More information about the role of the watchman may be found in Ezekiel 3 and 33. The photo below shows a watchtower reminiscent of Biblical times.