I saw this as a suggested video on YouTube yesterday: The Battle of Castle Itter: When the US and Germany Fought Together in WW2: https://www.youtube.com/watch?v=IRx7KkrT1QU (around 17:54 minutes long).
Wikipedia article: https://en.wikipedia.org/wiki/Battle_for_Castle_Itter.
Amazon book link: https://www.amazon.com/Last-Battle-German-Soldiers-Joined/dp/0306822962/ (not an affiliate link)
Atlas Obscura: https://www.atlasobscura.com/places/castle-itter
BBC article: https://www.bbc.com/news/world-europe-32622651
Google Maps: Castle Itter Google Maps
IMDB – The Last Battle: https://www.imdb.com/title/tt5138232/
Kickstarter campaign (ended December 21, 2018): https://www.kickstarter.com/projects/11606594/castle-itter-the-strangest-battle-of-wwii – not yet available on the Game company website: www.dvg.com Appears it can be pre-ordered on BackerKit: https://castle-itter-the-strangest-battle-of-wwii.backerkit.com/hosted_preorders