"Here are the two most important pieces of information you could possibly receive if you're planning on picking up Castlevania: The Dracula X Chronicles.
Play through the game to level 3' (you can do so by dropping into the first hole you find on level 2 and progressing through the level to fight the alternate boss.) In level 3', head through the level (heading up all stairs) until you find a skeletal snake. Kill it and destroy the hanging plant nearby, probably by using a subweapon, and then jump on the new platform to traverse the section on the upper level. When you get to the end, you should find a tombstone that contains an item which will unlock Symphony of the Night."