I get the feeling that the first Castle, was more like a transition between the physical realm and the Underworld, while the IC was actually underworld itself, which would explain why it reflects the first Castle. Galamoth in SOTN is described as a being with "plans to take over the Netherworld". Therefore if this wasn't the case, why would he be loitering in the IC? It seems to infer that the IC is at least easily accessible to the Underworld.
This fits with Shanoa's assertion that the Castle Keep is in Hell itself, and Arikado's assertion about the castle being a spiritual plane/world;
One detail: Galamoth's
original description in japanese says "一万年計画で魔界の王の座を狙う魔神 (Demon targeting the throne of the Makai in a 10,000 years plan)"
I don't actually think that the castle in SotN IS the demon world/underworld, because IGA said before that Dracula's soul is sent to the "Makai" (The same demon world that Galamoth wants to rule) after he dies. So Dracula in SotN is being summoned
from the demon world to Earth, therefore the castle can't be this demon world, as we see that Alucard stands between Dracula and Earth in the final battle. Dracula is not ressurrected, and Alucard is the one who is dragged into the portal and banishes Dracula before he can leave it.
But ruling over Dracula's castle probably is a symbol of "ruling over the demon world" as Dracula is the Dark Lord over all demons himself.
We're focusing too much on what we don't know about the castle, I think. What do we know, after all? Let's list it:
- The castle is a creature/extension of chaos and can take many incarnations (SotN)
- There is a spiritual castle, and a physical one. The spiritual one is the TRUE castle (AoS)
- It is a symbol of Dracula's magic (DoS)
- It's appearance coincides with Dracula's ressurrection (DoS)
- The Throne Room is needed to ressurrect Dracula (PoR)
- The castle can be shattered in at least two parts, which will cause both to be fake/transient until they are fused (HoD)
- It can ressurrect in different places (PoR)
- Cults can ressurrect the castle without Dracula (Ricordanza of the God Abyss)
- It can be brought back from the chaos inside humans alone (Ricordanza of the God Abyss)
All these details seem to say that the castle only becomes the TRUE castle once Dracula (or the designated Dark Lord at the time) sits on its throne and claims ownership of it. Until then it's "incomplete" or in a "transitioning state." Something like, if it's divided or incomplete, Dracula "aligns" it to become the real one.
So, maybe both castles in SotN are "fake" ones, until Dracula ressurrects or both are fused (which effectivelly means that yes, both are the same one, but each is incomplete, something you guys have already said)?