Adam Clarke on Acts 9:24

They watched the gates day and night to kill him - At this time Damascus was under the government of Aretas, king of Arabia, who was now at war with Herod, his son-in-law, who had put away his daughter in order to marry Herodias, his brother Philip's wife. As Herod was supported by the Romans, Saul's enemies might intimate that he was in league with them or Herod; and, as the gates of the city were constantly watched and shut, that no spy might enter, and no fugitive get away, they thought it would be easy to apprehend him; and doubtless got orders for the different officers at the gates to be on the look-out that he might not be permitted to escape.