Ancient Ruins
Go on your way to the northeast, get past the stairs allowing you to cross the river and a little further, you will meet the Owl that will inform you that they are two temples: one to the north and the other to the south and that you have to deal with the latter first. You will soon need a fifteenth arrows and therefore the Bow, so go get them if you didn't already.
Then go on to the east, then to the south. Push the Armos statues with your shield or hit them to wake them up and go on this way to the west, then get into the temple.
Go up to meet the big Armos Knight. To beat him, stay close to the door and right away shoot your arrows at him until the end (you need around twelve). You can also use Roc's Feather when he lands on the ground to avoid being stunned. On Switch, it's more complicated: While using Roc's Feather to stay up, load your sword with a spin attack and strike him twice to break his shield. After that, you can either continue this way, or shoot two arrows at him to defeat him. Then you can pick up the Face Key and go up. Light up the torches with some Magic Powder and read the writings.
Get out of the temple, and you meet the Owl again. Then leave the temple's boundaries, go west, look for a place where you can jump into the water south side, go south and climb on the small isle with two statues. Push the left statue, take the stairs, go through the underground tunnel thanks to the Hookshot and upon leaving go up (to the right). Place your key in its lock, then get in the...