castlecastle /ˈkæsəl/ noun 
► castle
► castle a large building with high walls to protect the people inside from attack, which was built a long time ago: The castle sits on a hill looking over the river.
► palace a large, expensive, and beautiful house where a king or queen lives: We visited the palace of the French King, Louis XIV.
► fort a strong building used by soldiers for defending an important place: The army began to attack the fort with cannons.
► fortress fortress is an older word that means the same as fort: They believed that no army could destroy their fortress.