(I suck at chess and im already stuck early on.), but I like the overall concept of this game!
I have some small suggestions:
1. Make it clear it is the king you need to send into the door. I didn't realize that for a moment and I thought I just send someone to the door.
2. Make a text to tell people 'R' is can always be pressed. It takes quite some time for me to realize that I can R other than dying. (Oh... it's written in stage 3... maybe earlier?)
Overall a cool game to me!