King Pig's Castle is a limited time event in which the goal is to pass five levels with a single life. 

Entering King Pig's Castle from the Fun Stuff menu or from the event button on the map screen will take you to a mini map with five levels. The goal is to complete all of those five levels without losing a life, in order to be able to open the treasure chest in the end. In case you fail one of the levels or you quit a level, your progress on the map is erased and you'll have to start over from the first level again.