The Mountain Castle, otheriwse known as Castle of Secrets, is a Snow Castle structure from the Better Dungeons mod. It is always a tall spire in a snowy biome, usually a glacier biome. At the bottom and outside of the castle is usually a vanilla village. This castle has 4 hidden rooms. There are books in the first two rooms that give hints on how to get to the next.
There is an opening in the main gate that can be closed right inside the gatehouse in the left courtyard. Walker Archers will begin attacking from the battlements as soon as you approach the castle. Once you are inside, there are several places to explore before heading up in either direction.
Work your way up the left staircase through the guard tower to approach the buildings on the hill. A bridge goes into the mountain where the officers keep warm.
Straight through all of the guards on the ground level, you'll find a village in the back through a shadowy path between rock overhangs. A skull on the ground marks a significant place.
From the courtyard you can find a narrow winding stair up the mountain itself. It will be guarded and you may be knocked off of the path. Wear your cloud boots if you have them.
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.
To the left of the skull in the lower village is a building with a mineshaft. follow the minecart rail and go thru the maze. At the end of the maze is a large room with the spider boss in it. The Spider Boss drops its signature spider hook.