When I was young and had a bunk bed it was pretty normal to box the lower bunk in with sheets, cushions from the couch — whatever — just to make it into a cave-like thing.
When I was a kid, I made pillow forts from the furniture in the living room. Fun times.
When I visited my brother this Christmas, he pointed out that this is an almost universal behaviour in young kids, which suggests that at some point in our evolutionary history we faced predators that specifically targeted small children.