translate English to German: The Korean movie Oldboy is often said to tell a vaguely similar story, as the protagonist is held prisoner for unknown reasons for 15 years.
Birgit Gottschalk: Das Kind von Europa. Zur Rezeption des Kaspar-Hauser-Stoffes in der Literatur, DUV, Wiesbaden 1995, ISBN 3-8244-4166-7 (zugl.