what is (a steep flight of stairs) in this sentence ? is it an idiom?
Down a steep flight of stairs in the basement of a bombed-out building lives more than 14,000 pieces of hope and inspiration.

Question

what is  (a steep flight of stairs) in this sentence ? is it an idiom?
Down a steep flight of stairs in the basement of a bombed-out building lives more than 14,000 pieces of hope and inspiration.

Answer

No, it's not an idiom. We describe a sharp upward angle as "steep". A set of stairs is called a flight.

The hope and inspiration are salvaged books.  The text is here:

http://nicol29ad.tumblr.com/post/149209369977/secret-library?is_related_post=1

Answer

In this sentence, 'a steep flight of stairs' is talking about the steep angle of the stairs that lead to the basement. It wouldn't be an idiom, although I think the 'pieces of hope and inspiration' is probably a metaphor for something.
Hope this helps :)