Malbork Castle — The political center of Prussia until 1466 was the Ordensburg Marienburg (Malbork Castle) in what is now Malbork, Poland

It is the largest castle in the world measured by land area and a UNESCO World Heritage Site.
Photo Credit: By Gregy - Own work, CC BY-SA 3.0 pl, https://commons.wikimedia.org/w/index.php?curid=21824061