When you hit an enemy, there MUST be feedback. It is unclear otherwise whether I have hit the enemy or not. For a while I thought it was one hit kill and I just missed, until I saw the +10% damage perk.
It was made worse due to the rock enemies with red eyes, which I assume can curl up and become invulnerable. In the first room I entered, I had both of them as enemies, so it was even harder to tell. In roguelikes, try to make the first enemies you face should not have any twists. Their purpose is to make it easy as possible to learn to core mechanics.
Also, I would suggest not to make "positive" thing look exactly like "negative" thing but different coloured. I just walked into the enemy spawning after the first room, because it looked exactly like the perk. Now imagine if someone were colour blind, they wouldn't be able to tell.
Hope my feedback isn't too harsh, good luck in future jams.