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 palace made its debut in Chapter 612.

Layout
The palace is covered with several layers of bubbles and the only way in is the connection corridor. The bubbles surrounding the palace are supposed to be impenetrable, but objects thrown by Vander Decken IX are able to penetrate them.

Princess Shirahoshi's quarters are in the Hard-Shell Tower (硬殻塔), which is in the northeast side of the castle. It was originally an armory where Neptune once locked himself in to calm his killing urges after Otohime was supposedly killed by a human.

There is also a prison tower where criminals (like Vander Decken IX and the officers of the New Fishman Pirates) are kept.

History
Years before the current storyline, Fisher Tiger had a meeting with King Neptune and asked him to join him, but Neptune refused.

Some time after Queen Otohime's assassination, Shirahoshi became a target of Vander Decken IX. Ever since then, Decken has been threatening her life. Shirahoshi has been locked in Hard-Shell Tower where she was kept safe from Vander Decken IX's Devil Fruit powers.

After the Straw Hat Pirates saved Shirahoshi's pet shark, Megalo, from the Kraken, Neptune planned to hold a banquet for them at the palace as an act of gratitude. Zoro was the first to arrive and Neptune soon brought Luffy, Nami, Brook, and Usopp as well as Keimi and Pappug. After arriving, Luffy wandered off alone and stumbled upon Hard-Shell Tower where he met the Mermaid Princess. Meanwhile, the Straw Hats were turned to suspects wanted for capture due to a misunderstanding caused by Caribou and Madame Shirley's claim of envisioning Luffy destroying Fishman Island. Roronoa Zoro was then locked up in the palace prison. The palace guards attempted to capture Nami, Usopp, and Brook, but with no success. Upon hearing the "party" getting started, Zoro broke out of the prison and reached the palace hall just in time to intercept an attack from Neptune.

In the ensuing battle, Neptune and the palace personnel learned the hard way that the Straw Hat Pirates are more than they bargained for. Usopp, Nami, Brook, and Zoro subdued and tied up Neptune, the ministers, and all the guards. Thus, the Straw Hats have "taken over" the palace. When the three princes returned, all the palace guards could do was cry for help. With the connection corridor closed, the princes were unable to enter and they had no choice but to listen to Zoro's demands of allowing the Straw Hat Pirates to leave peacefully. On Zoro's request, they left to find Robin, Franky, Chopper, and Sanji.

Soon, Vander Decken IX send Hody Jones' captive human pirates to infiltrate the palace by throwing them towards Hard Shell Tower. After they landed, Luffy brought Shirahoshi out of the tower by hiding her inside Megalo's mouth. Once the three left the palace, the human pirates blindly attacked anyone they see while trying to find the switch that opens the connection corridor to the palace. Zoro defeated them, but one of the pirates managed to reach the switch.

Once the entrance to the palace has been opened, Hody Jones and Vander Decken IX began their invasion. After learning that Shirahoshi is no longer in the palace, Decken left to go find her. Hody Jones then commenced his attack on the palace, injuring Neptune and some of the guards in the process. Nami slipped out of the palace with Keimi during the ensuing battle. Zoro, Usopp, and Brook released everyone before the palace was completely flooded with water. The palace guards and the ministers managed to escape the palace, but Neptune, Zoro, Usopp, and Brook were left behind as the palace fell under the control of the New Fishman Pirates.

Hody chains up Neptune and imprisons Zoro, Usopp, and Brook in a room with rising water. The New Fishman Pirates also find the letter Otohime acquired from the World Nobles and the box containing the citizens' signatures. Hody then broadcasts his speech to Fishman Island. He explains his plan of recreating the Ryugu Kingdom with himself as the ruler, which includes executing Neptune and the citizens who gave their signatures. He then addresses to the rest of the Straw Hat Pirates, saying that he intends to make examples out of them.

Hody then leaves the palace with Neptune and takes him to Gyoncorde Plaza. Zoro, Usopp, and Brook are locked in a cage in a room with rising water. Pappug later helps the three Straw Hats escape.

At some time during the battle at Gyoncorde Plaza, Caribou sneaks into the palace, releases the mermaids he kidnapped, but steals all the treasures in the treasure room. The guards eventually find all this (but Caribou had already left), and the Straw Hat Pirates had all charges lifted.

After the defeat of the New Fishman Pirates and Vander Decken IX, Hody and his officers as well as Decken are locked up in the palace prison. Later the royal family, the Straw Hats, and their friends have a feast as a celebration. Robin and Neptune then speak to each other in private and Neptune reveals the history of Joy Boy.

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 led here for saving Megalo, Princess Shirahoshi's pet shark.