After you visit Richard and retrieve the five Gold Leaves from Kanalet Castle, you find the Slime Key and can enter Key Cavern.
Get a Key
Throw a pot at the north door, then go through. Kill all the monsters in this room. When you hit the Bombites, they will bounce around the room and then explode. After all the monsters in the room are defeated, a chest appears. Open it to get a key.
Get a Second Key
Go north through the northeast door. Kill the four Green Zols that come up from the ground. This opens the doors of the room. You can open the chest, but it contains another Green Zol.
Go north to the next room. There are two Stalfos here. Kill them and go north.
Go north and be careful of the Green Zols hiding around the stairs. Go down the stairs.
Be careful of the four Green Zols that pop up around the stairs when you arrive. There is also a red Zol in this room. There are four locked doors. Go through the north one.
Kill the two Stalfos to get a key.
Get the Stone Beak
Hit the orb in here, then go back to the last room and go up the stairs. Open the chest to get the Stone Beak.
Get the Map
Go south from the room where you got the stone beak, kill the Stalfos and go south again. Kill the hidden Green Zols in the middle of the room, then go east. Walk over to the chest and open it to get the map.
Get a Third Key
Go west from where you got the map, kill the hidden Zols, then go north. Kill the Stalfos and go north again. Kill the Green Zols hiding around the stairs and go down.
Now go through the south locked door.
Kill all the monsters in this room to make a key fall. The Pairodd will disappear when you get close, but it usually appears in the same two spots, so when it disappears, run to its other spot and start swinging your sword so you hit it as soon as it reappears. If you are having trouble hitting it, you can try throwing a pot at it. There are also bombs under those pots if you want to try throwing a bomb at the Pairodd. Get the key that falls after you have killed all the monsters in the room.
Get a Fourth Key
Go north from there, then go east and avoid the Pairodd to make your way to the stairs. Go up. In the next room, go around and find all the Green Zols and kill them to make a key appear.
Get a Fifth Key
Go north to the next room, then go west. Kill the Zols to make the doors open.
Go through the west door and kill the two Pairodds to make a key appear.
Get 50 Rupees
Go through the one-way door in the north wall.
Kill all the monsters in this room and a chest will appear up above.
Go north and kill the Bombites and Stalfos. Jump up to get the flying bombs. Then use the stairs in the corner and go south.
Open the chest to get 50 Rupees.
Reach the Mini-Boss
Now go south, follow the path, and push on the block that is blocking the stairs. Kill the red Zols if they have reappeared, then go through the north door.
Mini-Boss: Dodongo Snakes
Put a bomb in front of the Dodongo Snakes mouths to make them swallow the bombs. After they have swallowed enough bombs, they will die, leaving behind a fairy. After both are gone, a warp spot appears.
Get the Pegasus Boots
Go east from the mini-boss room. Go to the right side of the room and push the bottom-left block to the left, then push the block above you up. Open the chest to get the Pegasus Boots. Equip the boots, then hold the boots button to dash. You can use this to dash through the green crystals at the bottom of the room, then push the block to the right so you can exit to the south.
Get a Sixth Key
Go back to the mini-boss room and use the warp spot. Go east and you reach a room where a Vaccuum Mouth pushes you away from the chest. Use the Pegasus Boots to dash toward the Vaccuum Mouth. Dash straight at it with your sword and you will kill it. The you can open the chest to get a key.
Get 200 Rupees
Go west to the entrance room, then throw a pot at the north door and go north. Go up the west path. There are crystals blocking your path, but now that you have the Pegasus Boots, you can use them to dash through the crystals. Do that, and go one screen north to where there is a Red Zol. Kill it, then go south to the room with the Bombites. Go northeast, then kill the four Green Zols to open the doors, then go north.
Kill the two Stalfos. Now that you killed the red Zol in the hallway to the west, a chest will appear in this room. It contains 200 Rupees.
Get the Compass
Now go back and use the warp spot to return to the mini-boss room.
Go south, then east, then north, then east, then north. You should be in a room with three Bombites. Hit the Bombites to light their fuses. After they count down to 1, they will flash and explode. After they explode, go around the room and kill any hidden green Zols you find. Then the doors will open. Exit to the west.
Open the chest to get the Compass.
Get a Seventh Key
Use a bomb on the west wall of the Compass room to open a hole. Go through.
Kill the three Bombites in this room to make a key fall from the ceiling. Take it.
Get the Nightmare's Key
Go back the way you came, to the room where there is an owl statue and an arrow pointing to a spot in the south wall. Use a bomb in that spot to open a hole. Go through.
Equip both the Pegasus Boots and the Roc's Feather. Dash toward the gap in the floor, then use the Roc's Feather to jump over the gap while dashing. You should go far enough to make it across. Go up the stairs and go north, then dash over the hole to reach the treasure chest. Open it to get the Nightmare's Key.
Get an Eighth Key
Go back to the room with the group of Red Zols that all move together. You should have four keys now. Use a key on the key block, then follow the path and use your keys to open all the key blocks. At the end, go down the stairs.
Use the Pegasus Boots to dash into the Thwomp and make it fall. Go east to the next room.
In the next room, equip both the Pegasus Boots and the Roc's Feather. Use the Pegasus Boots to dash, then use the Roc's Feather to jump while dashing. You should reach the ladder on the other side. Be sure to equip your sword again afterward. Get past the Piranha Plant and go down the ladder on the other side.
In the next room, use the Pegasus Boots to dash at the Pairodds and hit them before they can teleport. After you defeat them all, the door opens. Go through.
Ride the conveyor belts, being careful to avoid the Keese hiding on statues. When you kill all the Keese, a key falls from the ceiling. Take it.
Boss: Slime Eyes
Go into the Nightmare's Door. Inside, use the Pegasus Boots to dash into a wall. This makes Slime Eyes fall from the ceiling. Kill any Zols that might be in the room, then rapidly hit Slime Eyes's eye until it splits far apart. Use the Pegasus Boots to dash through the middle of Slime Eyes, splitting it into two. Hit the eye of each Slime Eyes. After you hit it, it will jump up, then come crashing down, stunning you. Equip the Roc's Feather and jump just before the Slime Eyes lands so you can avoid being stunned. Keep hitting the Slime Eyes until they die.
Get the Musical Instrument
Afterward, get the Heart Container, then go north through the door and get the Siren's Instrument: the Sea Lily's Bell.
After you get the instrument, you get a hint that the next instrument can be found behind a waterfall. Go east from the dungeon and the owl will tell you to go to Yarna Desert.