Was partly filmed at the real house of the Kaiser at Huize Doorn.
Upon arrival, Sigurd tells Captain Brandt to call the Kaiser and his wife "sir" and "madam", since the Treaty of Versailles prohibited otherwise. However, Brandt immediately calls him "Your Highness". This can be interpreted as WW2 Germans had utter contempt for the Treaty of Versailles and everything it represented.
Sigurd tells Stefan simply that "the Kaiser has a withered arm" but provided no details. When the Kaiser was born, he was a breech baby with his left arm wrapped behind his head. The nerves to his left arm were permanently damaged during his birth and it never properly developed. By adulthood, his left arm was six inches shorter than his right and his left hand withered and substantially smaller than the the right. The Kaiser would try to conceal this by holding a pair of gloves in his left hand to make his left arm appear longer.
Jai Courtney had to dramatically slim down for the role as he shot this film immediately after Suicide Squad (2016). He had gone up to 230 pounds for his last film and was sporting a very muscular and shredded superhero physique which the filmmakers thought would be completely inappropriate for his role as a war time German officer in this film. He also wouldn't have been able to fit into his tight military suits. Courtney had to perform a lot of cardio in order to lose his excess muscle mass in order to be ready by the time filming began.
The film marks the first time actor Jai Courtney has performed a scene of full frontal nudity on screen. Courtney revealed that he had absolutely no inhibitions about going nude. He also said that he didn't like the alternative of putting on a modesty pouch or taping his genitals to his body. He said about the scene, "I am pretty comfortable being stark naked. I actually didn't even know my genitals were going to be in shot. You can't put them out all the time but sometimes it makes sense to get real and get in the moment. If you trust the other actors and things are treated with sensitivity and respect, then it's fine."