Floor 5

Follow the hall to a large lava pit. Have Zelda enter the lava, then have Link hop on her back.

Move to the northern side of the lava pit and use the Whirlwind (a gust of wind created by Link) on the wind catcher there.

Move to the western side of the lava pit and use the Whirlwind on that wind catcher, being careful to avoid the Keese (bat-like enemies).

This action will open the door to the north.

Before proceeding through the northern door, dismount Zelda and open the chest nearby to receive a Big Green Rupee.

Proceed through the northern door and follow the hall. Ignore the turn to the north and you will find a locked door.

Head south into a room containing a single Spinut (a type of enemy).

Chase the Spinut into a corner. Have Zelda block one end of the corner while Link positions himself at the other. This will trap the Spinut, allowing you to defeat it and obtain the small key.

Use the small key to open the locked door.

Have Zelda carry you across the lava pit again, then head upstairs.