This part will take you through the entire Hyrule Castle.
The outside area
After Link reappears at the circular platform outside the front of the Palace of Twilight, enter the portal that will take you back into the light world to the Mirror Chamber. Have Midna warp you to Hyrule Castle Town, then enter the town and head north to the castle entrance from the fountain. Midna will then put together the Fused Shadow parts which will transform her into a multi-tentacled creature that will latch onto the Twilight barrier surrounding the castle and pierce it with a sharp pointed object, causing the barrier to disappear. Link now has access to the rest of the castle. Go through the first big set of doors and you'll see the glowing golden wolf that will take you back to the white misty place where the Hero's Shade will teach you the final hidden skill, once you've shown him that you mastered the Jump Strike. It is called the Great Spin Attack, which is activated the same way as a regular spin attack, but unleashes a more devastating blow to your enemies when your health meter is full. When Link learns the final hidden skill, the Hero's Shade will then depart for good, realizing that Link now has the skills to defend and protect Hyrule. "Now go and do not falter, my son," are the last words he will say.
After learning the final hidden skill, continue going north to the second set of big doors and use the A button to push one of them open. You'll now enter the outer court area of the castle grounds, where there are three doors: one going to the left, one going to the right, and a locked door going straight ahead. The ones to the left and right are both guarded by Bokoblins, and the outer court itself has Kargarocs flying overhead. Head to the door to the west, and you'll be surrounded by a Twilight barrier in which you'll have to knock down a horde of Bokoblins in order to disperse the barrier. Past the door, you'll have some Bulblin archers on towers shooting arrows at you, and a Bulblin that will attack you on foot. Take down the Bulblin attacker first, then shoot down the archers on the towers. Head all the way west until your way is blocked by a gate, then head north until you reach a passageway that goes east, then go that way until you see a boar you can ride on the break through the barriers. Take the boar to the western part of this area and ride north until you see two more towers with Bulbin archers on them. You can get off the boar and take down the archers or just simply knock them down by using the boar's dash attack. Past the towers are some pillars with propellers on top of them and a gate that must be opened by spinning the propellers. Using the Gale Boomerang, target the propeller on the pillar closest to the gate, then target the one on the pillar to the left, then the one on the pillar to the right, then the one on the pillar to the left near the south. When all four propellers are spun, the gate will open, leading you to a treasure chest that contains the Dungeon Map.
Go back to the outer court area and this time head for the door to the east. Again you'll have a Twilight barrier surrounding you, with a horde of Bokoblins to strike down before the barrier disperses. Go through the door and you'll enter the first part of the eastern area where a Twilight barrier will surround you and a horde of Bokoblins will come at you through a gate that opens in the east. Slay all the Bokoblins to make the barrier disappear, then head through the gate and go north until you come across another gate with a chain for you to pull in order to open it. Go through the gate and head for the northwest corner in the next section. A Twilight barrier will surround you as King Bulblin will appear with his battle axe, ready for a final match with you. Use the same strategy that you did the last time you fought him: use the Back Slice to get behind him and strike him until he turns around, then jump out of the way of his axe swings. Hit him enough times and King Bulblin will surrender and give you the small key before mounting up on Lord Bullbo and leaving. Midna will notice that this is the first time King Bulblin ever spoke.
The inside area
Head back to the outer court area and use the small key to unlock the front door. Enter the main chamber and you'll see a Twilight barrier forming around you before you are attacked by both Bokoblins and Lizalfoses. Take them all down to make the barrier disappear, then you'll notice as the camera pans around the room that a treasure chest will appear on a balcony to your left. Go to the top of the stairs on your left and Clawshot onto the chandelier hanging over that balcony so you can drop down and open the chest that contains the Compass. From the balcony, Clawshot onto the chandelier to the east and then aim for the one over the next balcony nearest you. Drop down from that chandelier and go through the door into the next room where you'll get into an enclosed-area battle with a Darknut. Use the same strategy as you did with the Darknut in the Temple of Time: circle around him and strike him when he's vulnerable to strip off his armor, then continue circling around him to defeat him. A treasure chest will appear on a high ledge in this room after you defeat the Darknut. To get to the chest, light two of the room torches with the lantern to raise up some ledges, then stand between the ledges as close as you can to use the Gale Boomerang to blow out the torch on the east end of the room. A platform will rise up below you so you can open the chest that contains 50 rupees.
There are two passageways leading to the east and west from the ledge where the treasure chest is. Take the east passageway and you'll enter a hall guarded by two Lizalfoses, which you can take out with a bomb arrow or two. The door they're guarding has metal bars over it, so you'll need to remove the bars first. Go into wolf form and use your senses to reveal some ghosts standing in a corner and pointing in a certain direction. Switch back to Hylian form and shoot a bomb arrow toward the peg above the picture in the corner. The metal bars over the door will now be raised. This will take you into another hallway where there are two big Darknuts for you to fight. Be careful and fight defensively when you're dealing with them, concentrating on stripping the armor off one Darknut first and then stripping the armor off the other. This will cause metal bars on two other doors in the hallway to be raised. Head for the door in the southeast corner facing away from the main chamber.
The outside area
You're now back outside where there is a walkway leading to a tower in front of you, and a walkway going to the right that will take you to a locked door and another walkway leading to another tower. Head straight forward to the tower in front of you, and you'll see the gate raised to a room containing a treasure chest. But you'll also see Lizalfoses charging out of the room, and Bulblin archers from windows in the tower firing flaming arrows at you. One of the arrows miraculously misses, though, and the Bulblin archers are shot down while the Lizalfoses are blown off the walkway. Link sees a hawk flying and then descending toward a group of individuals with weapons. This is Telma's resistance group giving you a hand at a time such as this. We can thank them later, but for now enter the room to open the chest to get the Big Key. Now head across the walkway going toward the locked door and this time head for the walkway going to the other tower. You'll get into an enclosed-area battle with an Aeralfos, which should be no problem for you to handle at this point. Enter the room in the tower and open the treasure chest that has the small key in it, then take it to the locked door and open it to reenter the castle.
The inside area
The room that you'll enter has blocks for you to jump across, but Link's movements will be slowed down again. Switch to wolf form and use your senses to see the Ghoul Rats clinging onto you, then shake them off using a Midna charge. You'll also see ghosts in the room standing on the blocks, pointing you in the direction you should go to jump across the blocks. Follow their directions so you can safely get through this room, then head for the broken staircase to jump across the gaps in the stairs. At the top of the stairs you'll get into an enclosed-space battle with some Lizalfoses, which you can easily take down. There's another set of stairs where most of it is gone, but there are some grated lights that you can use your Clawshots on to get across the gaps. Another enclosed-area battle with some Lizalfoses await you past that set of stairs. Past them is a third set of broken stairs with rails on them for riding the Spinner on, but watch out for the rolling spikes on the rails and jump across to the other side to avoid them. At the top of those stairs you'll see the Big Key door and a Darknut standing guard in front of it. Defeat the Darknut and open the door that will take you to your final confrontation with the boss of bosses.