The Tower of Cirith Ungol (Sindarin, "Spider's cleft") was a fortress on the left side of the pass of Cirith Ungol in the Ephel Dúath on the western border of Mordor. Frodo was taken into the Tower and searched by Orcs; however, he was rescued by Sam before they descended onto the Plateau of Gorgoroth. It is possible that the Tower of Cirith Ungol was originally called by a different name, because the pass of Cirith Ungol was previously called Cirith Dúath.
Drawings
Tolkien made three drawings of the Tower where only the turret is visible, and one with the entire bastion. This last picture depicts the Tower in an earlier stage of writing, with four tiers.
Description
The Tower stood against the eastern mountain face of the Ephel Dúath on the mountain slope on the left side of the pass of Cirith Ungol. It looked down on the trough between the Ephel Dúath and the Morgai and on the plateau of Gorgoroth beyond the Morgai. From the west only the round, topmost turret was visible. The turret was supported by three great tiers with pointed bastions that stood on a shelf in the mountain-wall far below and looked north-east and south-east into Mordor. The lowest tier was encircled by a thirty-foot high wall with overhanging battlements that enclosed a narrow paved courtyard. The main gate on the south-eastern side of the wall opened on to a broad road that ran along a precipice down to a southward bend and down to join the road that came over the Morgul Pass. A narrower path led down from Cirith Ungol by stairs to meet the broad road near the gate. In the gate were the Two Watchers.
Across the courtyard a great door at the foot of the tower led to its interior. A wide passage with doors on either side led from the door to the mountain-side. At the end of the passage was a great arched door that was the inner side of the Undergate. To the right of the Undergate was a winding stairway that led to the upper levels of the tower. The top of the stairs was covered by a domed chamber, with low doors facing east and west leading out onto the flat roof of the third tier of the tower. The roof was about 20 yards across and was surrounded by a parapet. On the western side of the roof stood the turret of the tower whose top rose high above the crest of the hills behind the tower. Another winding stairway inside the turret led up to the first and second storey of the turret. The turret had window-slits facing westward and eastward through which torchlight glowed like red eyes. In the ceiling of the passage, a trap-door led to the middle of the floor of a large round chamber at the top of the tower.
Third Age before the War of the Ring
The Tower of Cirith Ungol was originally built by the Gondorians after the War of the Last Alliance as an eastern outpost of the defences of Ithilien and of Minas Ithil. Its purpose was to defend Ithilien and Minas Ithil from attacks from Sauron's remaining servants by guarding the pass of Cirith Ungol and watching movements in Mordor.
Later the vigilance of Gondor failed and the Tower of Cirith Ungol was yielded up to the Lord of the Ringwraiths due to treachery.
War of the Ring
On 13 March 3019, Orcs from the Tower, under the command of Shagrat, captured Frodo after finding his unconscious body in the Pass below. They carried his body through the Under-way and into the Undergate that, besides the main gate, was the only entrance to the Tower. After overhearing Shagrat and Gorbag talking, Sam realized that Frodo had only been paralysed by Shelob and not killed as he had first thought. Sam was too late to gain entry through the Undergate and knocked himself senseless trying to get in.
When Sam awoke on 14 March 3019 he determined to enter the main gate to find Frodo. Mastering his fears, Sam came to the main gate and was halted by the malevolent will of the Two Watchers, stone statues flanking either side of the gate, topped with three vulture-faces each and endowed with spirits of evil. Holding aloft the Phial of Galadriel Sam overcame their will and thrust through the gate, but the Watchers let forth a shrill cry that was echoed by a harsh bell above.
In the meantime, the orcs had searched Frodo. When they found the shirt of mithril mail Frodo was wearing, a quarrel over the spoils erupted. A contingent of orcs from Minas Morgul led by Gorbag fought Shagrat and his company 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 proceed and rescue Frodo. Although Sam fought Shagrat, the Orc got away, bearing Frodo's gear to Barad-dûr. Sam finally found Frodo in the topmost chamber after killing Snaga. Sam returned the One Ring to Frodo, dressed him and gathered food and gear, and they left the tower. On the way out their use of the Phial caused the gate to crumble behind them. Again a bell clanged and the watchers issued a high and dreadful wail. This was answered far above in the darkness and out of the sky dropped the winged shape of a Nazgûl with a ghastly shriek.