I liked this one.
Jaynabonne said,
it could be "the mouth of a cave situated high up (in) the face of a sheer cliff"
I use,
"The opening of a cave is set in the middle of a shear cliff face."
The description of the exit or direction from the top could start with,
"Carefully peering over the edge of the cliff, you see the opening of a cave set into the side of the sheer face, far above the ground below."
Then something similar from the bottom....