The expansive Prague Castle is one of the most important cultural institutions in the Czech Republic. A UNESCO World Heritage site, this complex is also (according to the Guinness Book of World Records) the largest coherent castle complex in the world, with an area of almost 70,000 m².
Once the heart of the Holy Roman Empire, Prague Castle is now home to the Czech president and the crown jewels. However, the jewels are kept in a secret location, so you won't see them (which, while disappointing, is also pretty cool).
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Everyone must go through it (of course)