The Pass of Cirith Ungol
"The Cleft, Cirith Ungol, was before him, a dim notch in the black ridge, and the horns of rock darkling in the sky on either side."
Shelob's Lair
Gollum leads the hobbits to a long stairway cut into the living rock which winds away almost vertically up the side of the mountain. This stairway leads to a pass cutting through the Mountains of Shadow. The Tower of Cirith Ungol guards the pass, and there Orcs can watch for trespassers coming and going.
Frodo and Sam follow Gollum up the stair, up and up they climb. They have no other way to go, and yet whether they climb up or down both will lead to immense danger, Orcs below and something fearsome and unseen above.
They finally reach the top of the stair and all rest for a moment and to gather their bearings. Here lies the Tower of Cirith Ungol which is garrisoned by Orcs from Minas Morgul and Barad-dûr.
Frodo and Sam can see ahead of them a cavernous tunnel. This is the way forward.