I loved the concept as I am huge fan of prison movies! Also the gameplay is really unique and works well with the given theme. Moreover, I loved the art style and found the animation of the guard magnificent (ending).
The only thing that I have to say as a feedback is the combination of the gameplay mechanic and the story. Because I was focused on the guard not seeing me as I spammed the space bar, I couldn't pay attention to the writings. Maybe if you could show the letter to the player when they reach the end? I don't know!
All in all, great entry! Really enjoyed it!