This is really a comment to djm's answer but it got a bit long for a comment. Anyhow you should accept djm's answer.
The amulet you describe is indeed from the Chronicles of Prydain, though it's in book 2, The Black Cauldron, rather than Castle Llyr or Taran Wanderer. The scene where the overhang collapses is:
Taran, who had been thoughtfully fingering the clasp, stopped and looked about him uneasily. “Hurry,” he said, “we must leave here at once.”
“Taran of Caer Dallben,” Eilonwy cried, “you’re going too far! I can understand coming out of the rain, but I don’t see deliberately going into it.”
Nevertheless, she followed; and the companions, at Taran’s urgent command, untethered the horses and ran from the hillside. They had not gone ten paces before the entire slope, weakened by the downpour, collapsed with a loud roar.
The amulet was given to Taran by Adaon, though it's described as a clasp rather than an amulet.
Adaon shook his head. He put his hand to his throat and undid the iron clasp at the collar of his jacket. “Take this,” he said. “Guard it well. It is a small thing, but more valuable than you know.”