This one is really really simple. Load up any battle from any of the campaigns. Before you click the Start battle icon select the My Castle option which is just below the Equipment option. Once you are in the My castle screen Select Castle Editor by pressing Y button. Simply make a few alterations and make sure you save your new castle. Once the castle is saved click start battle and go on to win your battle and as soon as you win the achievement will pop. Naturally select a battle you know you can win easily and quickly.