I love the structure, the slow, difficult progression of the first half spent inadvertently memorising an environment that gets (literally) flipped in the second half. You don't get a map when you get to the inverted castle - you just have to work it out from memory and familiar cues.

I also love the weirdness of each individual enemy, and the sheer volume of them. It's an excessive game, the last gasp of 2D before 3D became king, which means it's actually dated rather well. It's boring to say that all roads lead to Dark Souls, but this is its most obvious stylistic influence - hands off, surprisingly stat-driven, and eventually able to be broken in an appealing way.