When Blazkowicz opens the secret passage he can overhear a torture interrogation about the whereabouts of Feldwebel (sergeant) Kübler. This is an in-joke poking fun at a plot hole in a movie made by a fellow student at the film academy.
The voice of the interrogated is that of 'SpongeBob SquarePants'.
The loudspeaker announcement in the castle courtyard orders SS Sturmmann (private) Günter Grass to report to the Schreibstube (that can either mean a writer's study or the registry office) immediately. This is an allusion to the German Nobel laureate who disclosed at the time that he was drafted into the military arm of the SS at the end of WW2.
When applying for filming permits at public locations the director only used the movie's subtitle "The Secret of Kottlitz Castle" for fear of being refused (a Nazi trash comedy likely being too controversial).