Tower of Cirith Ungol
Orcs from the tower brought Frodo there for questioning after finding his unconcious body in the pass below. Only after overhearing their conversation did Sam realize the Frodo had been paralyzed by Shelob and not killed.
When the orcs searched Frodo, they found the shirt of mithril mail that Bilbo had given him. A contingent of orcs from Minas Morgul was also present at the tower, and their leader fought with the leader of the orcs of Cirith Ungol for possession of the mithril. The fighting spread throughout the tower, and most of the orcs on both sides were killed, making it possible for Sam to enter the tower and rescue Frodo.
The main entrance to the tower was guarded by the Two Watchers.