i feel funny that developer knows the red hexagon doesn't look like a damage source, so he put one right in the begining with a healing orb so that player can learn it. I would suggest just quickly draw a pile of spikes for it.
the level design is alright. I think it's a good learning resource as it has dividied sections. from jump-focus sections to fight-focus sections, also a boss fight room. Even when the game is so small, i can still see the dev is trying to maintain a learning curve.
i look forward to your next game