noun
Palace of the Dragon King; dragon palace under the sea
Mythical palace at the bottom of the sea, famous from the Urashima Tarō folktale. Often used in set phrases like 竜宮城 (Ryūgū-jō).
浦島太郎は竜宮で乙姫様と楽しい時を過ごした。
Urashima Tarō spent a pleasant time with Princess Otohime at the Dragon Palace.
この海岸の沖には
Legend has it that there is a Dragon Palace off the coast of this beach.
竜宮城 is the full name for the Dragon Palace castle, often used in the folktale; 竜宮 alone can refer to the palace more abstractly.
From Chinese 龍宮 (lónggōng), meaning 'dragon palace'. The Japanese reading りゅうぐう is the on'yomi of the characters.