In part 1 of 'The Lady of Shalott', the very beginning and opening lines provide us with an atmosphere of mystery-- and all this mystery in fact leads to the Lady herself, and revolves around her.
Some of the more significant images in this ergard are:
1. The mysterious, cold and shivery breezes that blow about the locale, or setting;
2. The river itself, which like a moat or barrier, flows continuosly around the isle, and which itself is a rather murky and overgrown, mysterious place.
3. The small tower or castle of Shalott is also rather mysterious and somewhat prisonlike, 'embowering' i.e. keeping within, hiding, the Lady within.
4. The Lady's presence itself, too, is a mystery.
There are a number of similar images that reinforce the mystery, if we look at the text closely.