Adam Clarke on 2 Kings 9:17

A watchman on the tower - These watchmen, fixed on elevated places and generally within hearing of each other, served as a kind of telegraphs, to communicate intelligence through the whole country. But, in some cases, it appears that the intelligence was conveyed by a horseman to the next stage, as in the case before us. At this time, when the armies were at Ramoth-gilead, they were, no doubt, doubly watchful to observe the state of the country, and to notice every movement. See on 2Sam 13:34 (note).