Hyrule Castle is the teeny tiny castle where the teeny tiny King lives. It's the capital of Nowhere. Originally the castle was placed in Mushroom Kingdom and was named "Ludwig's Castle Hotel", but Mario invaded castle and shrunk it with toasters. Later Morshu bought the castle and gave them to King as present. King shrunk himself and put the castle in the center of Hyrule and named it "Hyrule Castle". But later it was moved west, because Squadala Man conquered Nothing Valley.
The King (who was the only survivor) built a second castle from ruins of The first one (which are ruins of Ludwig's Castle Hotel).