Whenever you come across a trapped cell or door, having a key in your hand will easily unlock them, but in Hollow Knight: Silksong, you will see many variations of the same keys. In order to free the Green Prince, you will again have to get your hands on the Simple Key to unlock the prince in Hollow Knight Silksong.
Where to Find the Simple Key to Release The Green Prince?

You will find a Simple Key located in the northeastern region of Sinner’s Road. Unlike the other time, here you must defeat an enemy to get the Simple Key.
From the Green Prince prison cell, climb all the way up and take the path right. Once you exit the room, climb up, and you will eventually face an enemy at the top of the arena. Defeating the enemy will drop a Simple Key on the ground.
Note: We have marked the spot of Green Prince (Green Square) and Simple Key (Orange Star Icon) on the map for easier finding.
You can then take the Simple Key and head back to the Green Prince prison cell down at the bottom region of Sinner’s Road. Interact with the cell and use the Simple Key to unlock and free the prince in Hollow Knight: Silksong.
Note: All Simple Keys work the same without being tied to a specific door. They all work the same; think of them as lockpicks.
Where to Find the Green Prince In Hollow Knight Silksong?
You will find the Green Prince at the bottom of Sinner’s Road. From the Bench at Halfway Home (Greymoor), climb up to the top and take the path right to get to Sinner’s Road. Here, you must follow the linear path until you exit the room on the right. You will find the Green Prince at the bottom, but unfortunately, he is trapped in a cage.
The Green Prince has a role to play later when you unlock the secret Act 3. You then get an opportunity to access Verdania and interact with him, complete his quest, and unlock a trophy.
The only way to unlock the Green Prince is by getting your hands on the Simple Key. Currently, Silksong has many Simple keys that can open many doors in the game besides the cell. If you already have one accidentally, you can use it to open the prison cell to help let out the Green Prince.