After you rescue all of the Seven Sages, you obtain the Triforce of Courage and can enter Lorule Castle to defeat Yuga.
Lorule Castle 1F
Go to the north end of the room and push the statue off of the right side. Merge with the wall and go east to reach a chest. It contains a red rupee. Then go back to the north end of the room and go through the north door.
Defeat the ball and chain soldier, then go through the north door.
Lorule Castle 2F
Go through the south door to the main room of 2F. Go to the south end of the room and merge with the wall, then go west to reach a chest on the left side of the room. It contains a potion ingredient. Then go back to the central area and go through the east door. Open the chest to get a red rupee. Then hit the two torches to blow out the flames, and unequip the lamp if you have it equipped. Go south and walk along the invisible walkway to reach a chest. It contains a silver rupee.
Go back to the large room of 2F. Go through the south door.
Lorule Castle 3F
Hit the crystal switch to create a green portal. You can use it to go back to the entrance and save if you wish. Go north and push a statue north to fall to the lower floor. Go north to fall where the statue went. Open the chest to get a key. Now go back to 3F.
Before proceeding, you can find a fairy fountain if you wish. Go to the east side of 3F, just above the stairs. Merge with the wall and go south until you are above the hole in the southeast corner of 3F. Pop out to go to a raised ledge in the southeast corner of 2F. Merge with the south wall and go west. Keep going until you reach a raised ledge. Go through the door there.
There is a door in the northwest corner of 3F that has bomb symbols next to it. Make sure you have bombs, then step on the switch and go through the door.
Go north and hit the crystal switch, then go west and hit the crystal switch from a distance so you are on the left side of the blue wall. Throw a bomb over at the switch and run south past the red tiles so that you are south of the red wall when it comes up.
Go west and then south and stand on the blue tiles. Activate the crystal switch from a distance so that you are standing on the blue tiles when they come up. Go south. Throw a bomb up at the crystal switch on the right while standing on the red tiles on the right. When they go up, go north and open the chest to get a red rupee.
Lower the red tiles, drop a bomb next to the switch and stand on the red tiles to the left. When they go up, go south. Throw a bomb to the right so it explodes next to the bomb flower. Merge with the wall to go around to where the bomb flower is. Hit the crystal switch to the right of the bomb flower to lower the red wall.
Now take a bomb flower with you. Go west and then north to where there are blue walls on the left and right. Throw a bomb at the crystal switch up above to lower the blue walls. Throw another bomb at it and run east past the blue tiles. Go up and hit the crystal switch up above. Carefully go northeast, protecting the bomb flower from enemies and the ice-spitting thing, and put the bomb flower in front of the big cracked rock. Open the chest to get a small key.
Hop down the ledge to the east and unlock the door.
In this room you have to fight Moldorm again. It's slightly more difficult because the walls sometimes move, making the room smaller. After you defeat Moldorm, step on the warp tile. The Moldorm symbol on the big door will light up.
Now go into the door that is east of the big door on 3F. In this lava area, go north and walk toward the lava. Some platforms will come up out of the lava. Use those platforms to reach the chest in the northwest corner of this area. It contains a potion ingredient.
Now go east from there. There is a ledge to the north that has a locked door on it. Go to the ledge east of that ledge. You find a statue that is blocking the path of a big spiked ball. Pull the statue to the left to allow the spiked ball to roll. When the ball starts rolling, get in front of it and run south to make the platforms come up under the ball. Then when you get close to the spiked ball south of you, run onto the platforms to the right. The spiked ball will hit the one south of it, knocking it into the lava. Go to where the spiked ball was and walk south to make more platforms appear. Merge with the tall platform to the east and then go south. You will reach a ledge in the south wall that has a door in it. Go through that door.
The tiles in this room will start flying at you. Stand on either the left or right side of the platform to raise a wall behind you, then face away from that wall and raise your shield to block the tiles from the other side. When all the tiles go away, the doors will open, a platform covers the lava, and stairs to the upper door appear. Go up the stairs and through the door.
Go north up the platforms that come up from the lava, then go east from the spiked ball. From there, go north to the northeast corner of the room, where a spiked ball is coming down. Go south along your platforms to make platforms appear under the spiked ball. When there is a wall south of you, merge with it and go south so you can make a platform appear before the spiked ball falls. Quickly pop out to the south and merge with the next wall and go south to keep the ball rolling. It will hit the other spiked ball. Now you can walk up the platform that the spiked ball was sitting on. Go north from there. Go as far north as you can so that you can reach the chest to the north. It contains a small key.
Now go south from where you got the small key in the lava room. When the platform path splits, go west. Make your way to the locked door in the northwest corner of the room. Unlock it and go in.
You have to fight a giant Barinade. This is more difficult than before because the platforms move side to side in the lava, putting you at risk of falling in. When you hit Barinade with your sword, you are knocked back, making it more likely that you will fall in the lava. Also, Barinade becomes electrified sometimes, so if you hit it then, you will get injured.
After you hit Barinade enough times, it splits into many small Bari. Hit these when they are not electrified.
After the battle, step on the warp tile to return to the big room of 3F. The Barinade symbol on the big door will light up.
Lorule Castle 4F
Go through the northwest door of the southern room of 3F to go to the next floor. Go east to find a chest that contains the compass. Now use your sword to put out the torches behind the chest where you got the compass. Two invisible railings will appear to the south. Walk only on those railings: there is no solid ground between them. To the south, you find a big chest. It contains the Red Mail. This cuts damage in half again. Damage will now be 25% what it was when you were wearing the Green Mail. Go north along those invisible railings, or just fall down to a lower level and then come back up here.
In the northwest corner of 4F, there is a door with lantern symbols in front of it. Step on the switch and go through the door. Go north and hit the torch with your sword to blow it out. An invisible path appears. Keep your lamp equipped. You won't fall, even though the path becomes invisible near your lamp. The path goes west from the torch, then north, but with the lights out, there is a barrier between you and the rest of the path. Go and light the torch that you blew out a moment ago, then walk where you saw the path going. Walk up to the next torch. Blow it out to see that the path goes east and then up to where there is another torch. There is a barrier, so you have to go back and light the torch that you just blew out. Go east along where you saw the path go. Don't light the next torch yet. From the torch, look west to see the path to the chest is, then light the torch and go along that path to reach the chest there. It contains a small key. Go back to the torch you just lit, then go north and go through the locked door.
You have to fight a ball and chain soldier on an invisible floor. After you kill it, step on the warp tile that appears. Another symbol lights up on the big door. Go back up to 4F.
In the northeast corner of 4F, there is a door with a hookshot symbol next to it. Use the hookshot on the wall switch to pull it down. Go through the door that opens.
There is a wallmaster in this room. Go east from the door and use your hookshot on the wall to the east. From there, go onto the moving platform. When it reaches the north platform, go north to reach a chest. It contains a purple rupee. You can see a switch west of this chest. Go south and use your hookshot on that switch to pull it down. This makes a platform to the south start moving. Step onto that platform. When you reach the two eyeball statues, go west and merge with the west wall, then go south. Wait for the wallmaster to fall on the cracked tile, then put the eyeball in the northwest corner of this platform. Merge with the wall and go north, then use the hookshot to pull the eyeball to you. Throw the eyeball into one of the statues.
Now go north onto the raft. Use the hookshot on the west wall to pull yourself over there, then use the hookshot on the north wall to be pulled up there. Pull yourself east and then wait for the wallmaster to appear. Merge with the wall to make it fall right away. It will activate the switch, lowering a wall. Pull yourself north, east, and north. Then go west, north, and west. Now you can walk off of the raft.
In the area to the south, break the skulls to reveal three switches. Push the statue onto one switch, then wait for the wallmaster to come down. Make it fall onto one of the other switches, then step on the third switch before the wallmaster can get up. Go past the wall that lowered, then stand under the cracked tile with the eyeball. Wait for the wallmaster to break that tile, then take the eyeball. Put the eyeball onto the raft in the lava. I recommend letting the wallmaster fall somewhere near you and then killing it with your sword, because this will cause a longer delay before the wallmaster reappears. Be very careful during this next part. If you accidentally use the hookshot diagonally, you might pull yourself away from the raft and have to start over. If you accidentally use the hookshot on the eyeball, you might get caught by the wallmaster. Use the hookshot to pull yourself back to where the eyeball statues are. Throw the eyeball into a statue. You can go outside and save the game: the eyeball will still be in the statue.
Open the chest that appears, then use the small key in the locked door. Down below, you have to fight Arrghus again. The battle is pretty much the same as before. Step on the warp tile. The four symbols on the door are lit up, so the big door opens. Go outside and save, then come back and go through the big door.
You'll be back in the room where you first emerged in Lorule. You can use the warp fissure in the north wall to go back to the corresponding room in Hyrule Castle. There are fairies there if you need them. The door there is closed, so you have to warp back to Lorule Castle with the warp fissure. From there, go east and follow the path into the Throne Room. Go north to find Hilda. After the cutscene, you must fight Yuga.
Yuga can disappear and reappear in a random location. Wait for Yuga to swing his trident, then hit Yuga when he is holding his trident to the side. Avoid standing in a line with the trident, because Yuga might thrust it toward you or charge forward in the direction that it is pointing. Yuga is vulnerable right after he attacks, so hit him then.
When Yuga gets weaker, he might attack with columns of fire. Just keep moving and you will avoid the fire. He might also throw the trident around the room, so run away from it. If it looks like he is about to throw it, stand near his left side so you can attack him in the moment after he throws the trident.
Yuga, phase 2
After you defeat the first form of Yuga, he becomes stronger. The battle starts with dead man's volley. Hit the energy ball back at Yuga until it hits him. If the energy ball hits the wall, it splits apart, so be careful. Also, if you don't hit it with good timing, it floats back toward you instead of going toward Yuga.
After the energy ball hits him, he merges with the wall. You will receive a new weapon that you can use while you are a painting. Equip it and merge with the wall. Shoot Yuga, then while he is blocking the arrow, pop out of the wall and merge behind Yuga, then shoot him from behind. He will pop out of the wall. Hit him with your sword until he warps away.
He makes flames shoot from the walls toward you, so run away from those. Then he will reappear in the center. Play dead man's volley with him again to make him merge with the wall. This time he will run while in the wall. When he runs toward you, pop out before he can hit you, then merge behind him while he is trying to slow down, and shoot him from behind. Hit him with your sword when he pops out.
After that, he will do another game of Dead Man's Volley, but this time using two energy balls. Afterward, he merges with the wall again. This time shooting him from the front doesn't stop him, so shoot an arrow away from him. It will go all the way around the room, hitting him in the back. He pops out, and so do you. Hit Yuga with the Master Sword to deal the final blow.
After Yuga is gone, go up to the painting of Zelda. Congratulations!