Scott Wise’s review published on Letterboxd :
If I were to plot this narrative arc on a graph of tension versus time, it would simply be a linearly sloping straight line with a hockey stick bend that arcs upward sharply to within a few degrees of vertical within the last five minutes. The premise is an incredibly simple one, but it’s in the near perfect execution of the film’s few set-pieces that the directing tag team of Balagueró and Plaza inject a truly bone-chilling experience. If this film isn’t an experiment in how far tension can be ratcheted up without any real release, I'd be surprised. I honestly can’t remember the last time I’ve been so riveted and yet so utterly terrified by a single scene in any movie before this one.
Manuela Velasco’s Ángela, the protagonist of this descent into chaos, is vibrant on screen. Equal parts brave and buoyant, her character is well-balanced and throughly compelling. Her desperate attempts to make sense of an incredible situation are tangible, and through the eyes (or lens) of her cameraman, it’s her struggles that make us connect with the situation that unfolds. Ferran Terraza’s Manu, a fireman who is a key character for much of the film, is also excellent on-screen, and one of the few characters on equal footing with Ángela and her motivations. The rest of the cast runs the gamut from superfluous to interesting but don’t feel like fully dimensionalized personalities worth spending more screen time with. That’s not necessarily a bad thing, as many of these characters, well, don’t stick around for too long.
The film’s biggest weaknesses are its two biggest conceits: the first being the point-of-view perspective that gets in the way of true immersion by feeling overly choreographed at times. But that’s the film’s double-edged sword: embracing this point-of-view affords the filmmakers a tool for selective revelation and obscuration of information, and yet it also inevitably exposes the craft behind it. That’s something ultimately forgivable, but it’s what holds this film back from being a much better - or at least less flawed - film.
The other major weakness is the rushed plot explanation of the final act, which gives us just enough of an answer to ponder, while at the same time drastically altering our understanding of what has been occurring for the majority of the film. It also introduces - without giving too much away - a hugely important asset to the heart of the film that demands a bit of a logical leap to put all of the pieces of the film together. Overall, this rushed “resolution" felt to me like a bit of a cop-out, but it’s hard to take a firm stance against this plot conceit because of the final sequence of events that it allows - a truly harrowing and unforgettable series of visuals that will leave you shaking. It’s in these final moments that both conceits find a reason for being, and while that logical leap may feel problematic while it unfolds, it winds up being something that will leave you thinking about long after the credit scroll - and in horror, it's the lasting impressions that matter most.