To be fair, DraculaXX doesn't have a Keep.
You seem to fight Dracula in an elevated section of SotN's Colloseum Stage (some tile even seem eerily similar, which is even weirder since SotN is supposed to follow Rondo, not DXX).
The tower you climb is either an illusion or some other weird thing. The tower itself never ends (if you keep on going through the stage, you can actually get lost and repeat areas if you're not careful where you're going).
Check it out! Isn't it similar at least in aesthetics & layout?

and

versus

As for why you can see the Castle within the "Castle" last stage? I think it's a design error. The designers probably didn't want to scrap a pretty background like that so they kept it. It was most likely supposed to go as a background for the Clock Tower stage (since the Clock tower is on the other side of the castle after a bridge, similar to the CV3 Clock Tower), but then realized they couldn't use it because all of the areas the Clock Tower stage used, had no open spots so you couldn't use it as a Vista for the castle at all.
Also, something tells me those stages weren't meant to be in that order originally, or that there was a plan to start, perhaps with Maria, on the opposite side of the stage and work your way in, or something. We'll never know...