A castle, in the original sense of the word, is just a fortified home. Typically some sort of tower like structure sitting on top of a hill, surrounded by a wall. Their function was mostly defensive, so they usually weren’t very big – it’s easier to defend a small place than a big one. The [Tower of London](https://upload.wikimedia.org/wikipedia/commons/thumb/0/06/Tower_of_London_%28Foto_Hilarmont%29.jpg/1920px-Tower_of_London_%28Foto_Hilarmont%29.jpg) is a typical example for how a medieval castle looks, but it’s a lot bigger than most of them.
With cannons becoming available in Europe, castles were no longer very useful since cannons could just blast through their high walls and towers. So they gave way to forts and fortresses (big forts): Instead of high walls, they use low and thick walls which can take cannonballs without crumbling. The [star fort](https://upload.wikimedia.org/wikipedia/commons/thumb/b/bc/Ciudadela_Jaca_Vista_Aerea.JPG/1280px-Ciudadela_Jaca_Vista_Aerea.JPG) is a very typical example for that.
At the same time forts became a thing, the residences of nobles were redesigned to be more spacious and pretty than the often cramped castles. For example, the famous [palace of Versailles](https://en.wikipedia.org/wiki/Ch%C3%A2teau#/media/File:Vue_a%C3%A9rienne_du_domaine_de_Versailles_par_ToucanWings_-_Creative_Commons_By_Sa_3.0_-_073.jpg) is called “chateau” in French. Similarly, [Schwerin Castle](https://upload.wikimedia.org/wikipedia/commons/2/2e/Schwerin_Castle_Aerial_View_Island_Luftbild_Schweriner_Schloss_Insel_See_%28cropped%29.jpg) in Germany is called “Schloss” in German, that it the word for cheateau.
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