The Mandate Encampment Outer Gate Key is one of the most important to find in Phantom Blade Zero, since it unlocks an entire region that's otherwise inaccessible.
Though you find the gate to the Mandate Encampment when you first arrive in the Village of Lu, the key is actually towards the end of the Pangzhen area (the game's 4th region), which you visit in its second chapter.
Exploring the west side of the town, you'll eventually reach Gu Wangyue, a Chinese opera singer boss. After defeating her, you can drop down the well in her arena, climb the ladder straight ahead, and you'll find the key to the Mandate Encampment on a corpse in the following room surrounded by puppet enemies.
It's on the main track through Pangzhen, so you'll definitely go through this room—just make sure to pick it up when you do so you can unseal the gate.
