Interesting post about Amnesia!

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In 1839 prussia was a german kingdom, so Amnesia takes place on german ground.
Prussia was grown by Friedrich III of Brandenburg.

I also remember some towns like Berlin, Brandenburg when reading a secret note.

The Amnesia Wiki says Castle Brennenburg is most likely in northeastern Poland or Konigsberg which is in what is now Little Russia:



Which means it's not even close to Germany. Without confirmation from the developers it's impossible to say for sure, but it does say it's deep within the East Prussian woods which would mean it's somewhere right of the modern Polish borders.