Wikipedia: The Castle in Malbork (Polish: Zamek w Malborku, German: Marienburg) is the largest castle in the world by area. It was built in Prussia by the Teutonic Knights, a German Roman Catholic religious order of crusaders, in a form of an Ordensburg fortress. The Order named it Marienburg (Mary's Castle). The town which grew around it was also named Marienburg.
The castle is a classic example of a medieval fortress, and on its completion in 1406 was the world's largest brick Gothic castle. UNESCO designated the "Castle of the Teutonic Order in Malbork" and its Museum as the World Heritage Site in December 1997. It is one of two World Heritage Sites in the region with origins in the Teutonic Order. The other is the "Medieval Town of Toruń", founded in 1231 as the site of the castle Thorn (Toruń).