After landing in the pool of water, the King of Red Lions senses that you are wondering about your present location. However, there is no time to explain: everything will be clear once you obtain the weapon located inside the castle. If you want a refill, you can chop down the grass near the pool. When you are ready, enter the castle.
After you’ve finishing looking around, it is time (pardon the pun) to get down to business. Climb down the staircase to get in front of the statue (from its point of view). Nearby, on the floor, you’ll see a Triforce marking.
Also nearby, you’ll find three triangular prisms. Once you manage to decipher the vague hint from the King of Red Lions, you’ll realize that you can drag the blocks onto the Triforce marking. You can only pull one corner at a time, so it may take a second to get used to - don’t worry, this is the only time you have to do this!
In this room, you’ll find a large pedestal surrounded by many glass windows depicting the six Sages, the Triforce, and Ganon. Proceed to the pedestal and the King of Red Lions will contact you on the Pirate’s Charm. He congratulates you because you solved the puzzle and says the here, before you, is none other than the blade of evil’s bane; the only weapon that can beat Ganondorf - it is none other than the Master Sword!
As Link struggles to lift the sword, the nearby statues lower their own swords and the Master Sword slides out with ease. The light from above suddenly grows brighter; the color returns to this land, indicating time has returned. It comes as a double-edged sword, however, as the enemies, too, have been revived.
The view will switch back to Link as he is swinging the Master Sword and then holds it up to the light. The message saying that you now have the Master Sword will appear, then you are free to leave - this room anyhow.
When you re-emerge into the main hall of the castle, you’ll notice that all of the monsters are alive and kicking… or stabbing… or slicing… or punching. VERY unfortunately for you, their are electric barriers near either exit, preventing you from leaving at all. This means you will have to defeat the eight-ish Moblins and the seven-ish Darknuts.
This will be a great aid, as you will likely fight multiple enemies at once. You know how to fight these by now, so I will not direct you. However, be sure to use the Grappling Hook to the Moblins’ Skull Necklaces and the Darknuts’ Knight’s Crests – especially the Knight’s Crest. I will make a small section about it in a second.
Once all of the enemies are defeated (there are some on the second level, too), the electric barriers will disappear. You could go through the unused exit, but leads onto a bridge.
About halfway across the bridge, you’ll come across an impenetrable barrier. Until the near-end of the game, you will not be able to pass through. If you did come this way, re-enter the castle and go through the other exit.
Outside, get back onto the boat and cruise into the ring of light.