How does Mary Stewart use imagery in The Hollow Hills?

Imagery:

The mist was lifting, drawing back from a sparkling sky. Faintly, high over the castle promontory grew a hazy moon of light. Then the last cloud blew clear, billowing before the west wind like a sail blowing towards Brittany, and in its wake, blazing through the sparkle of the lesser stars, grew the great star that had lit the night of Ambrosius’ death and now burned steady in the east for the birth of the Christmas King.

My very breathing sounded all at once too loud, an intrusion. I left the sword to its silent waiting, and went quickly back up towards daylight. The shadows parted and let me through.