Materials: KURA bed, mdf board, thin plywood.
I made a castle out of an IKEA Kura bed and two BILLY bookshelves.
I mounted studs at about 40cm apart for a durable upper floor. I placed a board (MDF, osb or thicker plywood board) in the correct size over the studs and attached it.
I cut a board that fits between the two Billy shelves and cut a half circle that will become the entrance for the bottom floor/bed. Then I screwed it on to the bed between the shelves.
I cut out a board that I can attach onto the side of the bed and leave an opening that will be the entrance to the upper floor.
I cut out the details out of thinner plywood and attached it. I used a piece of the thicker board to make the shield. Then I used filler(?) to cover the screws and then painted the castle.