Dude, this is great! So many ways of combining dash, jump and attacks to trigger different attacks or enable extra jumps! Really well done secrets! Especially love the one with the two hearts just before entering the castle. That kind of design is my favourite. Give the player all the tools and then, with a secret like that, show them that they CAN e.g. do double jumps. Figuring stuff out this way is so much fun.
The boss-fight is also well done. Very well tuned difficulty. Lots of fun, trying to use every movement-tool available to dodge the fireballs and to try to get close enough to land an attack.
I do regret going left at the start, though °_°