Cirith Ungol:

The Spider Pass, near to Minas Morgul on the borders of Mordor. Cirith Ungol was the route chosen by Frodo, on the advice of Gollum, to enter Mordor. It was a high, narrow pass overlooked by a fortress at the top. As an added safeguard, there lived Shelob, the greatest of the giant spiders remaining in Middle-Earth. The route through Cirith Ungol, included two very long stairways and a branching orc tunnel where Shelob lived.