Unlock this achievement by using catapults to kill 10 enemy soldiers at Prevant Castle Gate. You will have to make a choice between this and a Situation Bonus in which you destroy the gate in under 3 minutes because they cannot be done at the same time. Save the game and come back after unlocking the achievement. The easiest targets to hit are the enemies on the walls as they will not move and they are constantly replaced when killed. Move between catapults, because the reload takes longer than the time it takes to switch to a different catapult.