In the Akumajo Dracula X CD liner notes, the game's character designer Toshiharu Furukawa stated that they had to remove some monsters in previous Castlevania games, due to the game's international localizations. Having destructible monsters with humanoid forms (i.e. Medusa, Carmilla, and Gargoyle) went against America's morals. However, for Castlevania: Rondo of Blood, it doesn't need to be localized, due to it being released in Japan-only.
There are unused sprites in the game's memory for a giant lizard-like creature which shoots a beam from its mouth.
The track "Requiem," which plays on the file select screen, is derived from Christian liturgy. The phrase being sung, "Kyrie eleison," is Greek for "Lord, have mercy."
In stage 5, there is a secret passageway that can only be accessed by Maria. It leads to a white object resembling a mirror or doorway that cannot be interacted with. In the PSP remake, a Soundtrack item can be found here.
There is a hidden audio track message warning the player to not play the disc in a audio CD player. The warning is a small skit between Richter and Maria. Though only in Japanese, it seems that Maria initially scares Richter and Maria then warns him to not put the disc into an audio CD player. Richter questions why she knows all this before seeing she's been reading off of a piece of paper. Richter then warns people to turn down their speakers as it may blow them out before the disc goes onto track 2. A similar audio track is present on the disc Castlevania: Symphony of the Night.