The Catch achievement is another one which should come naturally as you play through the campaign, but you can obtain it easily in almost any game, so long as you’re in a siege. You’ll have a better chance if you’re defending against an attacking army, since they will be forced to attempt to breach the gates of your settlement. A Boulder can be found in a Stone Pile, which are often located on the ramparts near the main gates of a castle. All you need to do is pick one up and throw it down at the enemy, in the hopes that it will clonk one of them on the head and kill them where they stand. You only need to kill one enemy using this method to earn the achievement.