I liked how the film handled the backrooms themselves a lot more than I liked any of its organization into a proper film. The discovery, the exploration, the mapping, everything about that whole process was excellent and unnerving. But the drama and character dynamics felt schematic and the symbolism over-determined.
The dominant motif is iteration, with iteration giving a sense of endlessness, and therefore openness, but actually closing everyone off in loops that degenerate. The film applies the theme endlessly across its characters, storytelling, and visual style, such that I felt beat about the head. Just take the image of false freedom that dominates the movie, nowhere more obvious than the recurring gag of the window or open doorway seeming to show outside only for the sky and trees to be revealed as wallpaper. This is recapitulated on a visual level in every scene outside the backrooms (most rooms have small windows that rarely seem to look out on anything), and also becomes the final ambiguity the film is building towards: whether the windows in Reinsve's interrogation room show the sky or more wallpaper. It's recapitulated again in the story, with Reinsve using the window as her chief therapeutic symbol even as she's unable to lead herself or Ejiofor towards any freedom (including memories of a mother who wouldn't let her look out windows). The movie then reinforces this particular theme by iterating both copies of Reinsve's book and her tv commercial, books and commercials being, of course, mechanized iterations of images, ideas, identities, etc. This is all on top of the other big dramatic instance of iteration: Reinsve's therapeutic technique involves dramatic repetitions of crisis moments while she is herself stuck brooding on traumatic memories associated with two locations.
This is stifling. And, hey, maybe the proliferation of the same two or three symbols across all interpretive levels is a metacommentary on the backrooms themselves, the audience trapped and smothered by the film's own meaning-making system much as people in the backrooms are trapped in the detritus of their own memories. But that'd very conceptual and theoretical for a movie that, as plays, is practical and direct in how it communicates. Plus the point of the backrooms is that they feel both constrained and endless, where the film's visual and thematic overdetermination is only constrained. So densely theoretical or not, the stifling way of generating meaning wouldn't be a successful recapitulation of its setting even if that were intended.
Anyway, structurally the movie rests on a common fantasy trope, a doorway leading to magical realm that sits adjacent to our own.
Narnia is the big instance, but you find it in other works of popular fantasy like
His Dark Materials,
Little, Big, and even a recent bestselling fantasy novel whose world shares a lot in common with the backrooms as conceived in this movie (whose title I'm hesitant to name because me saying all this is kind of a spoiler). That this magical space is also a allegory for the human mind, especially its conceptual structures, is conventional, too. So Parsons has made a horror version of a common fantasy story, right down to the themes and the quest structure latent in the story (a gender-swapped quest to rescue someone trapped in a maze while pursued by a monster).
The movie is limited by the constraints of conventional characterization and storytelling plucked from fantasy. I'd like to see the concepts freed from those things and taken somewhere more unpredictable, because there's a lot of potential in the concept.