Malbork Castle, built in the 13th century by the Teutonic Knights, is the largest castle in the world by land area and a UNESCO World Heritage site. Originally constructed as a fortress for the Knights, it grew into a grand medieval stronghold, becoming the center of power for the Teutonic Order. Over the centuries, the castle witnessed sieges, royal ownership, and significant battles. One fascinating yet often overlooked feature of Malbork Castle is its innovative medieval heating system, known as a hypocaust. This system used a network of stone chambers beneath the floors to circulate hot air from furnaces, heating key rooms like the Knight's Hall. The hypocaust system, now largely forgotten, highlights the advanced engineering used to ensure comfort within the castle’s vast, stone structure during the harsh winters of Northern Europe.