What specific characteristics of the secret room keep the Nazis from discovering it?

Mr. Smit chooses the very top of the house in Corrie’s bedroom to build the secret room, because people will time to assemble during the search. During the next few days, workers sneak in supplies and build a brick wall, which is then painted and covered with bookshelves. Corrie inspects the wall, which has been water-stained and aged. Smit assures her that the Gestapo will never find it.