Remember, this is only what I know from my world, and it may be quite different here. I'd recommend asking Gilbert if you have questions, since he can explain, and he can do so much more easily than this.

[ He waves vaguely to indicate the general ghost situation ]

But I would guess that those doors could be used to call more of their kind, perhaps. There's a similar concept with the Abyss and with Chains, since there are areas in my world where the Chains are able to cross over more easily. It's not a door, but that's what I could at least compare it to. I do know that Katerina is doing this for the sake of revenge, and not exactly against a singular person. It would stand to reason that she would need more. But I also assume that these demons are the ones in control. If they're similar to Chains like I've been imagining, then they have their own goals here too.

[ and lo and behold, he was right... chains and demons are assholes ]