Wow, what an impressive game. This really nails all of the classic Metroid beats, escape scene included. I am getting vibes from a lot of other action platformers for NES like Mega Man here as well, and it blends nicely. The level design is well considered, especially the tutorialization within the rooms in which new powerups are attained.
So many little touches here that I love: The health pickups gravitate toward the player a la Metroid Fusion. The standard bullets careen off the doors like in Mega Man. The slide move glides down slopes kind of like Mario 3. The knockback on hits feels very Castlevania. It's a melding of many NES greats. Really well done!