The Two Towers Book 4, Chapter 3
They arrive at the walls of Mordor near dawn. High cliffs border the evil land, built up with walls and sentry towers. When day comes, a trumpet sounds and it is answered by bursts from the other towers. Sam is amazed by this and dismayed because he thinks that they should travel no further. Gollum tells them that they can go no further because they will be surely captured. Sam is silent. Smeagol says there is another way in and Frodo is frustrated that he did not tell them this before he led them to the main gates of Sauron. Sam thinks that Gollum is just trying to delay them until he gets a chance to take the Ring. Frodo thinks over the creature's claims and looks at the gate and becomes aware that there is a great commotion on the plain behind them. He can see the light bouncing off the spears and helmets of an army. A host of men is approaching the gates to join with Sauron.
Frodo tells Gollum that he will trust him, but reveals that he knows he is trying to get the Ring back. He warns him that he will kill him before that happens. Gollum is unnerved by the threat but grows calmer as he tells them of the other passage into Mordor. There is another gate at a place called Minas Ithil. It is guarded, but much less so than the main gates. He says that Sauron will not expect anyone to enter there. It is far to the west. Then he tries to convince them not to enter Mordor at all. Frodo accuses Gollum of not escaping Mordor but being let free to hunt for them. Gollum reacts harshly:
"'It's a lie!...He lied on me, yes he did. I did escape, all by my poor self. Indeed I was told to seek for the Precious; and I have searched and searched of course I have. But not for the Black One. The Precious was ours, it was mine I tell you. I did escape.'" Book 4, Chapter 3, pg. 318
Gollum is more sullen with the mention of Aragorn's name. He tells them that he knows the only way in and Frodo thinks of the words of Gandalf when he supposed that Smeagol would have a part in the whole affair. Sam watches the sky as the Nazgul fly over them. Frodo realizes that the enemy is looking for them. They linger in the shadows and Frodo considers their options. They hear singing and shouting. Squadrons of men move into Mordor and Frodo worries that some day all the men of the world will be under the control of the dark shadow. Sam asks if there are any oliphaunts, large creatures that hobbit legends place in the southern part of the world. Frodo doesn't see any and decides to follow Gollum to Minas Ithil.