Read it through once

Mrs. Fairfax stayed behind a moment to fasten the trap-door; I, by dint of groping, found the outlet from the attic, and proceeded to descend the narrow garret staircase. I lingered in the long passage to which this led, separating the front and back rooms of the third storey: narrow, low, and dim, with only one little window at the far end, and looking, with its two rows of small black doors all shut, like a corridor in some Bluebeard’s castle.