Great little puzzle! This is a great idea with lots of potential. I think you should make it playable on mobile by replacing the need to press number keys by buttons, it would be easy and would bring a lot of value to your game.
Also, some visual indicator of how many blocks you can still place would help, and leaving the blocks after the player dies would be more convenient, it allows to tweak your setup instead of starting over, which can be long. You could add a reset button in case the player really wants to delete all the blocks (ideally with a confirmation message).
I feel like you're on to something, great job!