Help me with this sentence?

"she conducts then to the most scenic route--------they must set off first outside the walls, on a rustic gry path through woodland, in the green belt that surrounds the historic site." i am puzzled by the sentence part "set off first outside the walls, on a rustic gry path through woodland, in the green belt that surrounds the historic site", though i understand the segments very well. i am not a native speaker. can some one rewrite the said sentence part in a plain way for easy understanding?