1A back gate or rear entrance to a building or site; often synonymous with 'back door'.
学校の裏門は普段閉まっている。
The school's back gate is usually closed.
犬が裏門から逃げ出した。
The dog ran away through the back gate.
工事のため、裏門からのみ出入りしてください。
Due to construction, please enter and exit through the back gate only.