Based on the British novel "The Nameless" by Ramsey Campbell.
The scenes in front of Claudia's door were filmed in the same building as [Rec] (2007), a movie from the same director.
Number #190 in Fangoria's Top 300 Horror Movies of All Time.
Carlos Lasarte, who plays the mad convict Santini in this movie, would later on star in Jaume Balagueró's movie [Rec] (2007).
Though it received mixed reviews upon release, the financial success of this film has been cited by film scholars such as Antonio Lázaro-Reboll for launching Jaume Balagueró's directing career and marking the beginning of a series of numerous studio horror films produced in Spain, from Balagueró's subsequent film [REC] (2007) and The Orphanage (2007), marking a "way of making films in Barcelona typified by quality genre production."