This game tends to be remembered more for the insane puzzle aspect of its endgame, but the more metroidvanias I play the more I appreciate the fundamental design elements of the first half in comparison to other games in the genre. A few in particular:

Level design is varied and distinct. Basically no two rooms look like one another. As you backtrack through the world how everything fits together becomes second nature. The genre is flooded with games filled with copy-paste rooms that can feel like a grind to get through, but in ESA you'll always be thinking "oh, right I'm in *this* room".

Consistently great bosses. Attack patterns are easy to read while still being challenging. These fights tend to feel more like puzzles because bosses have a fixed cycle of attacks rather than a list of moves done at random, so there's a nice tailored experience for how they play out, and the fights are well balanced so they don't feel like slogs once they're figured out.

The grapple hook is the best in the genre. It can't really be understated how much just this mechanic adds to traversing the world as you're left constantly wondering how to manipulate the geometry to get to places it feels like you shouldn't be in.

The metroidvania half isn't without flaws, mostly with how it's not always clear where you're supposed to go and the upgrades are fairly stock-standard for the genre, but the execution on the fundamentals really stands out and it's crazy think about just how much this game gets right even though it came out near the beginning of the genre's explosion in popularity in the indie scene.