I'm gonna just go ahead and call "all of the above" on this one.
I don't think it's really all one thing.
Drac definitely has a superiority complex and wants to be reachable ENOUGH that only those who can overcome his puzzles and traps and servants can reach him. By doing so, you have proven that you possess the brains and brawn enough to present an amusing fight; a trait in Dracula which absorbing Walter's Soul has doubtless magnified. The Castle is a creature of chaos, and so will often just put things in odd places, physics and logic be damned. There's also an argument to be had for them representing stairwells, particularly when they line up with the image of a spiral staircase in the background image, which happens more often than you probably notice (Circle of the Moon in particular was actually pretty good about doing this). And while I've never really considered "platform ghosts", it would make a degree of sense given the setting and what we know about it. And there are also moments where a given set of platforms is CLEARLY intended to link up with the Fourth Wall -- unless the Castle really IS the world's biggest dollhouse but only visible in this way from that one side the cutscene cameras never seem to face (unlikely XD).