Medieval Mediterranean naval warfare was therefore essentially coastal and amphibious in nature, carried out to seize coastal territory or islands, and not to exercise "sea control" as it is understood today.[159] Furthermore, following the abandonment of the ram, the only truly "ship-killing" weapon available prior to the advent of gunpowder and explosive shells,[160] sea combat became, in the words of John H. Pryor, "more unpredictable. No longer could any power hope to have such an advantage in weaponry or the skill of crews that success could be expected."[161] It is no surprise therefore that the Byzantine and Arab manuals emphasize cautious tactics, with the priority given to the preservation of one's own fleet, and the acquisition of accurate intelligence, often through the use of spies posing as merchants. Emphasis was placed on achieving tactical surprise and, conversely, on avoiding being caught unprepared by the enemy.