But just as he was about to pass under its great arch he felt a shock: as if he had run into some web like Shelobs, only invisible. He could see no obstacle, but something too strong for his will to overcome barred the way. He looked about, and then within the shadow of the gate he saw the Two Watchers.
They were like great figures seated upon thrones. Each had three joined bodies, and three heads facing outward, and inward, and across the gateway. The heads had vulture-faces, and on their great knees were laid clawlike hands. They seemed to be carved out of huge blocks of stone, immovable, and yet they were aware: some dreadful spirit of evil vigilance abode in them. They knew an enemy. Visible or invisible none could pass unheeded. They would forbid his entry, or his escape.
Hardening his will Sam thrust forward once again, and halted with a jerk, staggering as if from a blow upon his breast and head. Then greatly daring, because he could think of nothing else to do, answering a sudden thought that came to him, he drew slowly out the phial of Galadriel and held it up. Its white light quickened swiftly, and the shadows under the dark arch fled. The monstrous Watchers sat there cold and still, revealed in all their hideous shape. For a moment Sam caught a glitter in the black stones of their eyes, the very malice of which made him quail; but slowly he felt their will waver and crumble into fear.
He sprang past them; but even as he did so, thrusting the phial back into his bosom, he was aware, as plainly as if a bar of steel had snapped to behind him, that their vigilance was renewed. And from those evil heads there came a high shrill cry that echoed in the towering walls before him.
...
...

At length they came to the door upon the outer court, and they halted. Even from where they stood they felt the malice of the Watchers beating on them, black silent shapes on either side of the gate through which the glare of Mordor dimly showed. As they threaded their way among the hideous bodies of the ores each step became more difficult. Before they even reached the archway they were brought to a stand. To move an inch further was a pain and weariness to will and limb.
Frodo had no strength for such a battle. He sank to the ground. `I cant go on, Sam, he murmured. `Im going to faint. I dont know whats come over me.
`I do, Mr. Frodo. Hold up now! Its the gate. Theres some devilry there. But I got through, and Im going to get out. It cant be more dangerous than before. Now for it!
Sam drew out the elven-glass of Galadriel again. As if to do honour to his hardihood, and to grace with splendour his faithful brown hobbit-hand that had done such deeds, the phial blazed forth suddenly, so that all the shadowy court was lit with a dazzling radiance like lightning; but it remained steady and did not pass. Gilthoniel, A Elbereth! Sam cried. For, why he did not know, his thought sprang back suddenly to the Elves in the Shire, and the song that drove away the Black Rider in the trees.
'Aiya elenion ancalima!' cried Frodo once again behind him.
The will of the Watchers was broken with a suddenness like the snapping of a cord, and Frodo and Sam stumbled forward. Then they ran. Through the gate and past the great seated figures with their glittering eyes. There was a crack. The keystone of the arch crashed almost on their heels, and the wall above crumbled, and fell in ruin. Only by a hair did they escape. A bell clanged; and from the Watchers there went up a high and dreadful wail. Far up above in the darkness it was answered. Out of the black sky there came dropping like a bolt a winged shape, rending the clouds with a ghastly shriek.

(LoTR, VI-1)