As Link and the King of Red Lions descend through the portal, they enter a strange new world, completely frozen in time. Right in front of them is Hyrule Castle, the location of the true test. Upon arriving at the entrance to the castle, The King of Red Lions mentions an item is inside the castle for Link to obtain, and once he has it, all will become clear.
With those words of wisdom in your head, enter Hyrule Castle.
As soon as you enter the castle, you'll see several Moblins and Darknuts appear to be wreaking havoc in it. Or at least they would be...
Apparently, every single one of them is frozen in time! In this state, they're just harmless statues of their former selves. As you investigate the mystery of the castle further, you'll see portraits of the forgotten King of Hyrule, and the ancestors of Tetra and the Pirate Crew. However, most noticable is the big statue of the Hero of Time, Link's very own ancestor.
The Sacred Seal
Now, there's the matter of getting the item in the castle. Head straight down to the grand hall of the castle until you come to a place in the middle with three triangular blocks. Right there, the King of Red Lions will mention a puzzle that opens the entrance to the castle's basement, sealed to keep out evil. He also mentions a clue about how to open it, which seems to involve a familiar set of three golden triangles.
In order to open the basement, you must take the three triangular blocks you see nearby and arrange them into the shape of the Triforce. They're a little strange to maneuver, but a few pushes and pulls will get them into place. When all three are in position, the pieces will sink into their respective slots and form a complete Triforce, which suddenly glows bright yellow, like the gleam of the actual relic itself. With the puzzle solved, the statue of the Hero of Time slides back, revealing the entrance to the basement.
When you feel ready, descend the stairs into the secret passageway. After going through the multitude of trials and tribulations to find this place under the very seas themselves... the treasure that lies underneath here must be of prime importance.
The Master Sword
Inside the basement, you will find a strange chapel, and receive another message from the King of Red Lions. He says to take the sword you see in front of you. It is the legendary Master Sword, the only weapon that can defeat Ganon. Head over to its pedestal.
With a little bit of effort, Link grabs ahold of the Master Sword and pulls it out if its pedestal, causing it glow, unlocking its seal. The statues around him lower their swords in approval of a new hero, and the with a flash of light, the seal between their swords breaks. Now in Link's hands is the most sacred blade ever created, the Master Sword, weapon of the legendary hero.
Unfrozen, Yet Unfinished
With the seal over the castle broken, all of time unfreezes, bringing a normal appearance to its halls. However, every single enemy in the castle has now been freed as well, and now pose an actual threat to you. In its awakened state, a barrier blocks the castle's exits. In order to leave Hyrule Castle, you must defeat all of the enemies in the castle. Head back up to the upper floor of the castle to face them. As you leave the basement, it reseals itself.
Now, get ready for a long, grueling battle against the many Moblins and Darknuts in the castle. The Moblins are normal, but the Darknuts here aren't like the one you faced in the Tower of the Gods; their armor is silver, denoting a higher rank, and they have shields, allowing them to block more of your attacks. However, they share the same weaknesses the first Darknut did, and you can get rid of them quickly. Still, these Darknuts are creative. If you defeat a Moblin and disarm a Darknut, it may grab the Moblin's polearm and start swinging it around as a weapon.
Even though there are many enemies to face, now that you have the Master Sword, you can do more damage to them. If you can hold your own against these foes, you'll be able to walk out of the castle with the Master Sword, and several Knight's Crests and Skull Necklaces.