Ryugu Palace

The Ryugu Palace (竜宮城) is the royal palace of Ryugu Kingdom on Fishman Island. King Neptune, princes Fukaboshi, Ryuboshi, Manboshi, and princess Shirahoshi live here. It is apparently located in the smaller bubble that sits on top of the main bubble surrounding the island. The Straw Hats were invited here by King Neptune, but were turned to suspects wanted for capture due to a misunderstanding caused by Caribou; Roronoa Zoro was locked up in one of the prisons until he broke free. The palace made its debut in Chapter 612.

Princess Shirahoshi's quarters are in the Hard-Shell Tower (硬殻塔), where she has been kept safe from Vander Decken IX's Devil Fruit powers.

Trivia

 * Ryugu Palace is a reference to Ryūgū-jō, an undersea palace within the Japanese legend of Urashima Tarō. While the legend says that Tarō was led there after saving a sea turtle, the Straw Hats were lead here for saving Megalo, Princess Shirahoshi's pet shark.