The world's largest castle by floor area is a shopping mall in South Korea - Apparently.
The Lotte World Tower in Seoul, South Korea, occupies over 1,800,000 square meters, surpassing the floor area of the largest castle in Europe, Malbork Castle in Poland. Completed in 2016, this modern castle-cum-shopping-mall features over 300 stores, an ice rink, and a theme park. Its sheer scale makes it an unlikely yet remarkable example of a "castle" in the modern era.