Thank you for playing my game and for writing such a long message! In fact, I was still developing the game just one hour before the deadline. I wrote the code for showing the "E" key prompt, but I ended up forgetting to actually add it to the game. TAT; I originally wanted to add more enemies, some bosses, and even treasure chests, but I ran out of time. So I submitted the project in the final 4 seconds: I only figured out how to publish a game on itch.io at the very last moment...