Castlevania: Dracula X - Stage 4: Dungeon (Saving Maria, Alt Path to Stage 5')
19If you're trying to get the Good Ending of this game, you'll want to go through this level, Stage 4, instead of what's the game makes you think is the alt version, Stage 4', which is the Caves.
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You should still have the Key by this point. You want to still keep it for as much as you can throughout this level. Near the middle of it, there will be a big door. Stand as close to it as you can and then use the Key to unlock it.
From there, you'll find Maria, one of the two maidens you needed to rescue. Maria will thank you for saving her, but then mention that Annette/Annet, her older sister and Richter's girlfriend, is still trapped somewhere further ahead.
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The room in 4:45 is the last traveling part of this level. If you go all the way to the left, you'll then end up encountering a boss, but we don't want that if we want the Good Ending.
Hopefully, you still have the Key by that point. Just use it on the door to the right to finish the level, and completely ignore its boss in the process!