Hyrule Castle has become the stronghold of Calamity Ganon. You must go there to destroy Ganon and bring peace to Hyrule.
Activate Central Tower
Before approaching Hyrule Castle, it might help to activate Central Tower so you can see Hyrule Castle and the surrounding areas on the map.
Going Straight to Calamity Ganon
It is relatively easy to skip most of Hyrule Castle and go almost directly to the Sanctum where Calamity Ganon awaits, but Hyrule Castle is full of treasure, so you might want to explore it. It is your choice.
If you want to skip ahead to the final battle, just go to the waterfall in the southwest of the island that Hyrule Castle is on, put on your Zora Armor, and swim up the waterfall. Keep going up each waterfall that you reach, and you will soon be very close to the Sanctum. You just need to get past a few Guardian enemies and run into the building.
Entrances to Hyrule Castle
Hyrule Castle Town Ruins
The ruins of Hyrule Castle Town are directly south of Hyrule Castle. They are patrolled by Guardian Stalkers, though, so you might have to do a lot of fighting to get there. Once you go north from Hyrule Castle Town Ruins, you can use Magnesis to open the castle gates.
One of the entrances to Hyrule Castle is West Passage. This section explains how to get there.
East of Ridgeland Tower, there is a road that goes past Zalta Wa Shrine. Go east from there and you will cross Carok Bridge. As soon as you cross the bridge, go north and float to the island beyond a small river.
Walk along the west side of the island and look at the rocky hill to the right. Eventually you will see a wall that you can bomb. Bomb it to find the Noya Neha Shrine.
There is a Guardian Stalker on the eastern side of this island. Go to the northeast corner of this island where the road ends, but watch out for the Guardian Stalker as you go.
You will find a raft at the north end of this island. If you don't have a Korok Leaf, just cut down one of the smaller leafy trees on this island until you get one. Then go to the raft, use a bladed weapon or arrow to cut the rope, and then use the Korok Leaf on the sail to go toward the castle.
In the water, go south in the moat but approach the west side of the castle. You will eventually see a metal rail in part of the wall above the water. Sail there. You can enter a door into the castle at either end of that rail. If you go into the southern door, you can follow the path to find a rail cart. There is a bombable wall in this room that leads to the outside. Or you can get into the mine cart, which you can propel by putting a bomb into the rear compartment.
If you ride the mine cart north, after you reach the north end of the rail you will be in an area that has a small pool of water. This is a hot spring, so get in the water to heal up. Then you can go up the ramp that is lit with torches and you will reach a cracked wall. Bomb it to access further areas of the castle, such as Lockup.
East Passage can be accessed from the north of the castle. The castle has several black and magenta spires surrounding it. Go to the base of the northernmost spire and glide down to the ledge on the left. There is a Decayed Guardian in this area, and to the west of the Decayed Guardian, there is an entrance to East Passage.
Be aware that East Passage has a Stone Talus in it. If you climb up the shaft above that Stone Talus, you will find a passage blocked by an eyeball. Shoot the eyeball and you can go in to reach the Library.
The Docks can be accessed in the same way as the East Passage: go to the base of the northernmost spire around the moat and glide down to the ledge on the left. Instead of going west from the Decayed Guardian, go east and look for an archway in the stone wall with water flowing under the castle. Swim into the castle from there and you reach the docks. Light the large brazier in this area to reveal the Saas Ko'sah Shrine. This gives you much quicker access to the interior of Hyrule Castle.
Areas Within Hyrule Castle
Hyrule Castle is large and has many distinct areas within it, which will be named on your map when you go there.
The Library is accessible from the Docks. Go up the many stairs until you reach a metal wall. Use Magnesis to push it out of the way and you will be in the Library. Or you can go to East Passage and climb up the shaft above the Stone Talus to reach the Library.
Be sure to look for additional magnetic shelves in the Library to find more hidden areas (one of which contains a Stone Smasher), and read the books in the library to find some recipes.
If you go outside of the library via the north exit, climb onto the hill that the exit is part of, and use an Octoballoon on the large slab that covers the treasure chest. The chest contains a Naydra's Scale.
If you leave the library via the lowest south exit, you will see a chest across the way. It contains a Shard of Farosh's Fang.
The King's Study is behind one of the metallic shelves of the library. It contains a Royal Guard's Shield.
Path Between Library and Armory
The path between the Library and the Armory contains a Great Frostblade.
There is an Armory between the Library and the Dining Hall that contains a Moblin wielding a Royal Claymore, and a rack of weapons with a Royal Guard's Shield, a Royal Claymore, a Royal Bow, a Royal Halberd, and a Royal Guard's Bow.
Corridor between Armory and Dining Hall
The corridor between the Armory and Dining Hall has several walls that you can bomb. Two of them have pots hanging from the ceiling that you can shoot to find a Korok.
One of the hidden rooms contains a Royal Guard's Sword.
The Dining Hall is in the southeastern part of the castle. There are four Moblins there, but you can sneak in and take some food without them noticing.
There is a Royal Guard's Spear in the fireplace of the Dining Hall. One of the Moblins wields a Silverscale Spear, and another wields a Stone Smasher.
The Guards' Chamber is in the southwest part of the castle on the upper floor. There are Lizalfos and a Moblin here. There is a Knight's Halberd and Knight's Claymore near the Lizalfos. The Moblin wields a Royal Guard's Claymore, and behind him one of the statues has a Royal Claymore.
The Guards' Chamber has a gate to the outside that can be opened by stepping on a switch. You will need to put a barrel onto that switch to keep it open. From there, you can find an eyeball that you can shoot to remove a Malice barrier from the slope leading up from the Hyrule Castle entrance.
The Guards' Chamber also has a wall of dry leaves under the stairs. Burn them to find a chest that contains a Shard of Naydra's Fang. There is also a Moblin who wields a Royal Guard's Claymore. There is also a Savage Lynel Shield in a chest hidden behind some crates under the stairs in the area with the Moblin.
If you go north from the Guards' Chamber, you reach a barrier of Malice Goo. The Glowing Eyeball is in the ceiling just beyond the door.
North of that, there is a Glowing Eyeball in a room to the left. Shoot it to clear the stairwell that leads outside. The stairwell contains a Royal Guard's Shield and a Royal Broadsword.
Go north from there and you can either go outside to a tower with Princess Zelda's Study at the top, or you can go downward to a room that leads to Lockup. The room down below has a Moblin who wields a Royal Guard's Spear.
This room is in the south of Hyrule Castle on the upper floor. It overlooks the entrance from Hyrule Castle Town Ruins. There is a Moblin there that wields a Royal Halberd. Shoot the Glowing Eye to open the windows. There is a Thunderblade near where the Glowing Eye was. With the windows open, you can go out on the balcony, get the Royal Bow there, and climb up to the roof to reach more areas of the castle.
Lockup is accessible from the West Passage. There is a large chamber in one end of Lockup that has a Stalnox in it. The first time you defeat this Stalnox, you get a Hylian Shield. The Stalnox also drops a Thunderblade, Royal Spear, and Royal Broadsword.
Princess Zelda's Study
At the top of a tower on the west side of the castle, you will find Princess Zelda's Study, where you can read her journal. You can only reach this by climbing the tower.
Princess Zelda's Room
Cross the bridge outside of Zelda's Study to find Princess Zelda's Room. The bridge is a location of one of the photographs for the Captured Memories quest. You can read Zelda's Diary in her room.
On top of Hyrule Castle, you will find two Guardhouses. If you go into one of these Guardhouses, the gates will lock and you will have to battle a Lynel.
There are also Glowing Eyeballs in these Guardhouses that control mouths that spit Cursed Skulls at you, making the battle even more difficult. You can shoot the eyeball in the First Guardhouse without fighting the Lynel if you float in from an upper window, shoot the eye, then float to another window to escape. You will have to run far away from the Guardhouse, though, because the Lynel can shoot Shock Arrows at you from a large distance.
In the Second Guardhouse, there are two eyeballs, and you can't shoot the eyeballs except from the floor of the building, so you will have to battle the Lynel while dealing with Cursed Skulls.
The Second Guardhouse has a chest on the roof, and if you climb the spire above that, you will find a Korok.
The Sanctum is the highest room of Hyrule Castle, where Calamity Ganon is marked on the map if you select the "Destroy Ganon" quest in the Adventure Log. You can climb the outer walls of the Sanctum to find treasure chests and a Korok. When you enter the building, the final battle will begin.