OPT:
(a). This will be sturdy enough to support the weight of your kids' books, arts & crafts projects, or other materials. You can use cardboard boxes with thick sides, cardboard boxes with narrow sides, or boxes made from cardboard.;
(b). The shape should be rectangular and means the box won't leave a lot of space for ink to bleed into the pages. Make sure that the cardboard box is good for painting.;
(c). It should be large enough that it is easy to stack into a box. If you are handy about collecting the boxes, you could cover the bottom with cardboard and glue on two of the sides.;
(d). Cereal boxes, tissue boxes, or shoe boxes would work as well. Also collect four cardboard rolls, these can either be toilet paper or paper towel rolls depending on the size of your castle.;Choose from options above and answer: What most naturally follows?

How to build a castle out of cardboard boxes
Find suitable cardboard boxes.
A sturdy, nicely shaped box would be best. A good example is a box used for printer paper.
Answer:
(d).