Ryugu Palace

The Ryugu Palace (竜宮城) is the royal palace 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.

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.