Story Idea:
A Series of novels/novellas. Each one focuses on a different monster’s castle.
Wolfman’s castle only exists during the full moon. It is an ordinary rock formation that transforms in the full moon.
Invisible Man’s castle is totally invisible, only things that are added can be seen.
Frankenstein’s Castle is electric based
Dracula’s Castle is legendary, don’t screw with it.
One of the castle’s is inverted underground and is a cave essentially.
Have a monster hunter hero that investigates each one.
D&D style, so the hunter gains experience and equipment from each quest. Like playing Castlevania.