In response to the OP: I would think of the castle bed assembly as a group of smaller parts that can stand alone, but together they are the castle bed.
So as far as drawing up the castle bed, start by measuring the mattresses that you will use and the room/walls. In your mind think of the space the lower bed needs. Will the castle be located against a wall or in the center of the room? Against a wall will make the back side easier to construct because no one sees the back side of it. Placing the mattresses with the heads over one another, build a box over them that has the structure to support the upper bunk and an adult's weight (to make the bed). To build the box and structure, draw each side of it. ie, the head board side, the foot board side and the the two long sides will have two more drawings. These walls may actually need two drawings of the inside and the outside.
You will also need a drawing for the support structure of the upper bed and walkway. That drawing may have to be two views (one from the top and one from the bottom). The lower bed occupant may not like looking at a bunch of 2 x 4's above. (I think that view would be like a ceiling in an actual castle.)
Then you can start with the towers. Again picture what each side will look like and then draw those views out. (The towers could be built like a kitchen pantry cabinet, assuming they be used for storage). Then do the same for the stairs and the slide. Both of these could be straight items instead of the curves ones shown in the picture. Maybe the curve slide could be purchased. If so, buy that now so you can incorporate its dimensions into your drawings for the height of the upper bunk.
I use AutoCad 2007 for my drawings. But I understand the program from my engineering background. However, the drawings I have written about, can be simple pencil drawings on a pad of graph paper or lined paper. The main thing is to break the areas down into small pieces on paper. Then work area to area making sure they will fit up as you see fit.
Another thing to think about: Maybe some of the areas can be simple to start with and then each item could be reworked with more detail. Example: the towers could be a simple cabinet box with a face frame on them. Later add doors or drawers to them. Maybe a ladder now instead of the stairs. But make it safe for the users.
With all the various people here, there will be as many designs. Good Luck with this project.