Jamieson, Fausset and Brown on 1 Samuel 22:3

3. David went thence to Mizpeh of Moab--"Mizpeh" signifies a watchtower, and it is evident that it must be taken in this sense here, for it is called "the hold" or fort (1Sa 22:4). The king of Moab was an enemy of Saul (1Sa 14:47), and the great-grandson of Ruth, of course, was related to the family of Jesse. David, therefore, had less anxiety in seeking an asylum within the dominions of this prince than those of Achish, because the Moabites had no grounds for entertaining vindictive feelings against him, and their enmity, to Saul rendered them the more willing to receive so illustrious a refugee from his court.