inherent vampire superiority complex demanding to provide human challengers a path to the big man, albeit a tough one, to see if they're worth a fight
Pretty much what I think too.
And it's not just convenient floating platforms. Dracula's entire castle on all games is structured in such a manner that it is possible to reach him, instead of just locking all doors permanently, or placing the strongest enemies early, or flat-out sealing Dracula's room altogether. When someone MAKES it impossible to reach the throne, it's a vampire with no relation to Dracula who thinks he is a failure of a vampire... and then the castle screws him up anyway by producing a vampire-vaccine-ex-machina;
Dracula drew power from Walter's soul, so I wonder if Walter's attitude/personality stained his magic somehow that it was unable to build an impossible-to-overcome castle when Dracula created it.
Here is a another I just thought up: The castle is a creation of chaotic origin, and chaos is a product of humans' negative traits -- to the point where it's stated that the castle is summoned from humanity's heart itself.
So, what if the castle can't be made inexpugnable because humanity's heart
in itself isn't impervious to invaders? It IS a negative trait of humanity that it's not impervious to forces that wish to invade it without its consent. For instance, we have Shaft mindcontrolling Richter or Dracula controlling Soleiyu, amongst hundreds of others controlled by his curse in CVIII, and other millions more of humans exploiting and manipulating other humans. Maybe the flaw through which a human gets "invaded" is added to the "recipe" of Dracula's Castle when it is summoned. A perfect castle would be one that cannot be invaded -- hence this castle cannot be perfect, because it's literally made of humanity's imperfections. In other words: The castle is an ironic monument to humanity's failures down to its failure to protect its master.