Malbork Castle

It is sometimes referred to as 'the largest heap of bricks north of the Alps'. Under continuous construction for nearly 230 years, Malbork is actually three castles nested in one another. The High, Middle and Low Castles are separated by additional dry moats and towers. It housed some 3,000 "brothers in arms". The Low Castle walls enclose 52 acres (210,000 m?), four times the area of Windsor Castle.

More info at  en.wikipedia.org