Adin knew that he was the one who must fill the medallions in the belt.
Palace of Del is in need of more information! Palace of Del is lacking More sensical formation and a complete synopsis. Also, please make it clear that anime-only (and therefore non-canon) information is non-canon.
"In olden times, Deltora's royal family had lived there in splendid isolation. Now the palace was a refuge for people damaged in body and mind by the Shadowlands invasion."
— Shadows of the Master
The palace of Del is the home of the monarch of Deltora and their family. It is a large, three-story building, built by the Ralads by order of King Brandon. It stands on a hill in the centre of Del and can be seen from every part of the city. Surrounded by a large wall, the entrance is well guarded and is the only way into the palace besides secret tunnels which only the Royal Family know about.
By order of King Brandon, the palace was to be constructed and built by the Ralads. When the first floor was complete, Brandon moved into the palace, leaving the traditional home of The Royal Family, the forge, which had been the home of the heirs of Adin since the uniting of the Seven Tribes. The palace was finally completed after 40 years of building. It was built out of local sandstone and roofed with slate. The floor and internal pillars are made of marble. In the formal rooms, the ceilings are decorated with painted scenes, by Withick, of Deltora's past, the ceilings of the Great Hall and the Banqueting Hall featuring borders of pure gold.
During the Shadow Lord invasion the palace was his main headquarters in Deltora and home to his servants. The Place of Punishment was built, just outside, to be used for public executions of criminals.
After the restoration of the monarchy, the palace became a shelter for damaged Shadowlands survivors. 
The library is located on the third floor, and can be accessed by the great stairs. The door to the library is located in a hallway at the top of the stairs, to the left. The area is barricaded by a thick silver rope. To the right another hallway runs to the back of the palace, and is also barred by a silver rope. The right side of the hallway is lined with the library storerooms, the last of which was burned by Josef years ago. The library is filled with books, papers, and documents. Both important, secret, stories, and knowledge is located in the library. The library of the palace of Del, always has a librarian, and the first one we heard of was Josef the Palace Librarian in the time of King Alton, in the time of King Lief. Josef's days ended in the middle of the time of King Lief when he was killed by the poisonous song of the Sister of the South, which were located in the middle and the heart of Del. He was then replaced by Marilen, the cousin of Lief. The book The Belt of Deltora was hidden inside the library by Josef and later found by Jarred.
A secret tunnel only known by the royal family leads to the chapel of the palace. The entrance of the tunnel is camouflaged as a large rock appearing as a sleeping bear. The entrance is at the front feet of the bear is located (in front of its nose and face) in the earth. In the earth a little lid or plate of metal is located, and you have to open it to be able to get to the underground tunnel which is very small. You come out in the chapel, out of a hole, opened by removing a stone tile above your head (in the floor of the chapel).