Jerrin accidentally uncovers the Royal Hidden Passage in the emergency shelter of Lookout Landing.
Cave Location
You can enter the Royal Hidden Passage as soon as you land in the Hyrule Kingdom. To do so, go to the Observation Room in Hyrule Castle, at -0253, 0766, 0098. There is another entrance that you can reach by going down the stairs in the Observation Room and going to -0325, 0798, 0075. After you complete one of the four Regional Phenomena quests, an entrance to the Royal Hidden Passage will open in the Lookout Landing Emergency Shelter, at -0253, 0114, 0009.
Royal Hidden Passage Map
Cave Features
From the Emergency Shelter, break through the boulders, then break the boulders on the right to find a demon statue (part of the quest Who Goes There?).
Then break the boulders to the north. The path splits into two, with a tunnel that goes northeast and a tunnel that goes northwest. Go northeast to find some ore deposits. Go northwest and you will find a Like Like. There is a cave behind the boulders in the south wall that has some Sticky Lizards in it.
Then go back to where you defeated the Like Like and use Ascend to get on top of the ledge that the Like Like was on. Break the rocks here, and you will find some brown boulders against a brick wall, and blue boulders in another wall. The blue boulders take more hits to break.
The west path leads to a stone room with a Like Like in the southern part of the room. Defeat the Like Like and open the chest that was behind it to get the Soldier's Greaves.
There is also a chest in the northwest part of the room behind some boulders. It contains a Soldier's Claymore.
The path with the blue boulders leads to a bridge with two Stalblins and Stalmoblin. If you get in the water and swim east, you find a tunnel with some Glowing Cave Fish and Tireless Frogs.
Back on the stone bridge where you fought the skeletal monsters, break through the blue boulders to the north. There are some blue boulders on the side of the path that lead to a few ore deposits. The path eventually leads to a very large cave with a lot of luminous ore deposits and a big pile of boulders in the middle. If you break all of those boulders, you reveal a Stalnox.
There are black boulders east of the Stalnox. They will break if you throw three bombs at them. This tunnel leads to an area with an Ice Like. After you defeat the Ice Like, open the chest behind where it was to find the Soldier's Helm.
Then you can go north from the Stalnox room and break through the blue boulders just beyond the water. There will be an area to the left with Luminous Ore. Before you go down below, look in the east wall for some vines in front of a small tunnel. Cut down the vines, then shoot a fire fruit at the next set of vines in the tunnel, then crouch to get into the tunnel, where you will find some mushrooms. Exit the small cave to return to the area with Luminous Ore. If you go down below, you can break the Luminous ore in the east wall to reveal a cave with three Horriblins.
Go west to exit the Horriblin cave, go back up above to the paved path, and continue going north past more walls of blue boulders. You will reach an area with boulders against a stone wall. Break them down and defeat the Stalkoblins in the room at the end, then lift the boulder in the corner room to find a hole in the floor. Go to the other end of the tunnel and use Ascend to reach the room on the other side of the bars. Defeat the Stalkoblin and open the chest behind it to get the Soldier's Armor.
Fall back into the hole, go through the tunnel, then use Ascend on the ceiling next to the hole. Exit the stone room to return to the cave. There is slope to the east that leads to a wall of boulders up above. Break the boulders and the path will split. You can optionally go to the west to find a room with roots and mushrooms and bomb flowers. To the north, you find a Shock Like. After you defeat it, go through the tunnel in the north wall behind where the Shock Like was. You will find this cave's Bubbulfrog, and some ore deposits.
Back in the room where the Shock Like was, go up the roots to find a tunnel to the north. Shoot a bomb at the Luminous Ore up above, then climb the north wall to find another tunnel, where some Stone Pebblits will attack. Break the boulders up ahead, and you will reach a spiral staircase. The first hallway from the bottom of the staircase leads to a room with a Horriblin and a chest that contains a shield. The second hallway leads to a Horriblin that is guarding a chest that contains a halberd. Beyond that, the path leads to a broken ladder with a trap door at the top. Use Ascend to go through the trap door and enter Hyrule Castle. You can then open the trap door with Ultrahand.