If you loved Das Geheimnis hinter der Tür, try Das Testament des Dr. Mabuse
Eine Brücke zwischen einem Film, den du schon gesehen hast, und einem, den kaum jemand kennt. Das teilen sie, und was der zweite macht, was der erste nicht macht.

Das Geheimnis hinter der Tür

Das Testament des Dr. Mabuse
Was sie teilen
Both films are directed by Fritz Lang, and they both carry the paranoid mood tag, and they sit in Mystery / Thriller territory. If that's the register that drew you to Das Geheimnis hinter der Tür, the second film will land in a comparable space — through a different lens.
What Das Testament des Dr. Mabuse is
A lunatic asylum east of Berlin, dawn frost on the windows. A patient scrawls invisible orders while keepers hum hymns. Detective Lohmann wakes in a padded cell, watched by a colleague who speaks in riddles. Professor Baum claims the criminal genius Mabuse has been dead for years yet his rambling treatise still steers new heists across the city. Lang’s Weimar shadow-play where ink bleeds into blood and silence howls.