Where are these Fairies?

Hint 1 of 11:

There are two entrances.

Hint 2 of 11:

Have you noticed a cave anywhere?

Hint 3 of 11:

It looks obvious, once you see it.

Hint 4 of 11:

It's behind the waterfall.

Hint 5 of 11:

But there's a rock in front of it.

Hint 6 of 11:

Have you seen Gwystyl yet?

Hint 7 of 11:

He gave you something intangible.

Hint 8 of 11:

Use it.

Hint 9 of 11:

If you saved Hen Wen from The Horned King, then just find her pig-tracks.

Hint 10 of 11:

Notice that they stop by the edge of the lake.