It's fantastic, in the sense that it's full of unbelievable things happening, but it doesn't really follow many (if any) of the trops and conventions associated with fantasy as a modern genre that we know and love.
I agree that those gothic tales are much more
proto-fantasy than modern fantasy. They introduced many genre tropes that are now part of modern fantasy -- and D&D in particular -- but they also lack many tropes central to the modern genre.
So, they include dark, mysterious, and
immense architecture, and they also include the supernatural, but they take place in dark corners of our real world, not in a mythical prehistory or parallel world.
Although they're steeped in certain medieval elements, they don't borrow much from medieval
romances or Norse sagas, and they're not at all Tolkien-esque.
When I read it, I experienced the first (original feels derivative). Also, many scenes struck me as unintentionally funny.
Yeah, I'd expect a bit of an
MST3K experience.
Of those, I've only read Frankenstein, which is a book that I like very much. So many movies have been made of it, but the movie makers have failed to understand the book IMO.
Honestly, I hated
Frankenstein, but I loved
Dracula.
Interesting bit of
Frankenstein trivia: the original text never describes Doctor Frankenstein constructing his monster out of human cadavers.
But even above-ground ruined castle chambers feel very 'dungeon-like' if the ceiling is intact; I've been in many such in Aberdeenshire.
It's
hard to build an immense structure full of twisty passages with much natural light. Once you rely on artificial lighting though, that concern vanishes, and complicated structures don't have to be dark and dungeon-like.