Th
At the top of the mountain is the real city where the royals live. There is a lot of loot and there are a lot of Diamond-Armored Walkers. There is also a secret room not mentioned in the mysterious books. It is fairly easy to get lost in here, and a large number of walkers spawn (10-15).
One floor underneath the market/farm room you will find a room surrounded by bookshelves and a table. If you mine the bookshelves behind the table you will find a room with 2 double chests. If you mine on the opposite side through more bookshelves you will find another hallway, if you follow this hallway you will find the Walker King and his royal chamber.
After killing the King, enter his room, on the right side of his bed you can find a room by mining through the stone blocks as shown in the "Mountain Castle Interior" Picture with the bed.
Boss
To the left of the skull in the lower village is a building with a mineshaft. Follow the minecart rail and go through the maze. At the end of the maze is a large room with Shelob, a giant spider. The Shelob drops its signature Spiderhook. The is also the Walker King that can be found in the Upper Keep guarding his chambers.