Where to Find the Backstreet Courtyard Key in Phantom Blade Zero

The Backstreet Courtyard Key in Phantom Blade Zero is obtained in the Mountain of Imprisonment area by speaking to the couple near the Shrine Outer Gate, completing the Ledger of Atonement quest by finding the ledger in the caves below, and then speaking to the woman there to start the Bai Xue quest. The key is mainly relevant to that specific quest and holds little value beyond it. Reaching the Mountain of Imprisonment requires completing the Illusory Pipa quest first.