Maybe the guarding forces were confused.
Wolfman: Wait, what are we doing again? Are we guarding Dracula's castle in the inverted forest, or that Richter guy's castle in the regular forest?
Skeleton: Peeping Eye said Drac told us to guard at the inverted forest.
Wolfman: Are you sure? Won't Alucard be coming from the other one? I thought the only way here as the portal behind the throne room where Richter is waiting?
Skeleton: Yes I'm sure! Guard the inverted forest! That's what P.E. heard!
Wolfman: He's a freaking eye with a tail! He doesn't have any ears! I don't think we should take his word.
Skeleton: Look, if you wanna face Drac's wrath, be my guest. I'm gonna stay here and guard the inverted forest.