Japanese castles filled much the same role Western castles did, providing defence and housing for feudal nobility. Unlike castles in Europe, the design relied on high sloping walls on the bottom floors and took advantage of Japan's mountainous landscape with elevated placement that gave castle dwellers the ability to better monitor their territory.
Look at reference pictures of real Japanese castles to get an understanding of their layout. Stack boxes of assorted sizes to plan your castle, with at least one large box at the bottom and smaller boxes towards the top. The first layer of large boxes will be your base.
Cover boxes in white paper, to cover previous branding and marketing. Where possible, conceal seams, so that the coverage appears to be one piece of paper. Use glue and a brush, or wrap the boxes like gifts. Allow all glue to dry before continuing.
Using navy blue card paper, such as cardstock or Bristol board and a black marker, draw a wavy grid to cover one side of the paper to represent a tiled roof and roll the card paper loosely so that the tile pattern is on the inside, to create the familiar curve of traditional Asian roofs.
With pieces of paper covered cardboard or white card paper, create the sloping lower wall around the bottom of your castle. Cut triangle pieces sized to fill in corner gaps. Be sure to leave one rectangular space open for the front gate on the lower level.
Using grey or beige and brown markers, draw rounded stones to scale along the outside of your lower wall. Tape together on the inside.
Create the appearance of a pagoda roof by trimming the pre-decorated pieces of curved navy cardstock to fit the boxes. Two pieces of curved paper -- placed so they make an inverted V -- make a whole roof. Tape together, again on the underside, where it doesn't show.
Using glue and a paintbrush, secure the levels of the castle together. Allow them to dry completely before moving on.
Using markers in various colours, draw in the main gate, windows and decorative railings. Reference photos will give you historically accurate images to copy.
The open nature of the plan allows you to adapt this to fit existing models or miniatures. The smaller the castle you want, the thinner the paper and cardboard you should use. Decorate with model trees and figurines.
Take care with sharp scissors and craft knife to avoid injury.