To unlock the 1997 PSX classic Castlevania: Symphony of the Night, you must head to the alternate version of the third stage in the 3D remake. Once there, head upward to the topmost level of the stage, where you fight a bone snake that's attached to the wall. Just left of the bone snake, hanging from the ceiling, is a green cage-like object you need to hit (Maria can easily hit it with her normal weapons, Richter may need an Axe subweapon). Hitting the green cage creates a platform that lets you jump to the upper row of platforms. Continue right along this upper row of platforms until you reach a white tomb stone. Destroy it and collect the pickup to unlock Symphony of the Night. (Note: You do NOT need to complete the stage for the unlock to take.)