Another easy way to get this if you missed it on the first level, or can’t be bothered looking for every Nazi in a level, is by simply completing the final level in episode 1 (9th floor).
*** Spoiler - click to reveal ***This is the boss fight and you only have to kill him and then make your way to the exit, escaping from Castle Wolfenstein. Note: there is a secret room behind one of the Swastika banners about halfway down the left-hand red brick wall. The room is full of extra ammo and health to help you kill the boss.