what is the best way to build the walls?
Answer from Piñata Boy
Whenever I’m making a piñata with a flat surface I look at using thin cardboard. Applying papier mâché over thin cardboard will give you a stiffer wall so it doesn’t bend outward when the piñata is filled with candy. Depending on how big the piñata is, you might even need to use thin corrugated cardboard instead. Store-bought piñatas are often made of corrugated cardboard, which makes them notoriously difficult to break, but you can weaken the walls by stabbing holes in them before you decorate. This should leave the walls structurally sound, but still breakable.
You can get large sheets of thin cardboard and thin corrugated cardboard for free from Sam’s Club or Costco. Just pull them out from between the stacks of toilet paper or paper towel and they will let you take them home for free.
One thing to be careful of when applying papier mâché strips to cardboard is that if the papier mâché is too wet, the cardboard can absorb moisture and become wavy. So be sure to squeeze out excess moisture from the newspaper strips before laying them down.
Depending on the design of the castle, you may be able to save yourself some time by finding some flimsy cardboard boxes to build the general shape of the castle out of. This would result in a piñata that has more than one candy compartment inside.