Thanks for the input, Exkyo! I’ve decided to tweak the black cells—they are meant to be late-run allies, but to make the progression clearer, I'll allow players to unlock the upgrade skill much earlier.
To clarify the damage mechanic: health is lost when shapes hit the walls. While the demo currently keeps you on one map, the final game will transition you to the next level immediately after defeating a shape. Maybe we should show it with numers anywhere in the screen?