I tend to come off as a bit critical of Symphony of the Night. While Super Metroid's design seems near-flawless even today, Castlevania has a tendency to treat its players unfairly and leave them lost in deep dungeons, pit them against enemies far beyond their own power level, or leave them without clues in an ever-increasing problem domain. I'll go through some of these flaws in this analysis, but first I must acknowledge that the overall aesthetic of Castlevania is quite different from Metroid's. The planets you explore in Metroid are always hostile, but never menacing. Castlevania's less thoughtful design does capture the feeling of being trapped in a living, breathing labyrinth filled with unthinkable treasures and beasts from every mythology; that atmosphere can certainly be amplified by a sense of being lost and not knowing the way forward. Symphony of the Night is much less methodolical in its progression than Super Metroid, which is often frustrating but also kind of fitting.
Time to get started. For such a large game I was surprised to count the number of locks and keys in Symphony of the Night. There are a total of seven upgrades required to make it through the game, and only four to get the bad ending. One of the main reasons for this is that the game lets you play through a 1-2 hour linear corridor before offering the first key. Here is the area accessible directly from the start of the game: