Same two characters as 门道, the knack, but here the second syllable keeps its full fourth tone. This reading is the literal one, a physical passage. If the sentence is about skill or know-how rather than a building, the reading is 门道.
usage
This names the covered passage of a gate, the stretch between the street door and the courtyard in an older Chinese house. 门洞 names the same space and is what most people say, so this reading turns up mainly in descriptive writing.
