Sumptuous is probably overused, especially when describing things that are either Gothic or giallo, but this is both. It earns the designation, from the architectural porn to the high fashion (interwoven into character, setting, and plot) to the long hallways and tight interiors to the wallpaper, yes, the glorious, sometimes garish, sometimes sublime wallpaper. By god, what wallpaper. (I love the wallpaper.)
As a murder mystery, it uses the "kill the suspects" model, especially toward the end, a sort of lurid Agatha Christie story with a supernatural twist and grotesque secrets. But giallo is not entirely about the plot; someone is being killed and someone is killing. Like the blunter cousins in the slasher genre, these films are sometimes more…