New Modes in Symphony of the Night - After clearing the game as Alucard for the first time, you can start special modes by entering a specific name when you select a new game.

Password: Result:

X-X!V''Q Play as Alucard, with 99 luck and the Lapis lazuli (+20 luck) accessory
AXEARMOR Play as Alucard, with the Axe Lord armor
MARIA Play as Maria Renard
RICHTER Play as Richter Belmont

Unlockables

Dracula X: Rondo Of Blood: Unlock Castlevania: Symphony Of The Night

When the minotaur chases Richter in Stage 2, immediately fall into the first hole to enter the sewers. Complete the level as usual by defeating the Boss. You will then start Stage 3', the alternate version of Stage 3. Every time your path splits in Stage 3', always select the upper route. Collect the Axe subweapon when it becomes available. Use it to cut the vine-covered cage near the skeletal snake that is attached to a wall. Jump on the cage to allow Richter to jump to the upper ledges. Continue along the upper ledges until reach a breakable tombstone that contains the item that unlocks Castlevania: Symphony Of The Night as a playable game at the main menu.
Dracula X: Rondo Of Blood: Play as Maria

When the minotaur chases Richter in Stage 2, get the Key subweapon from the candlestick. Go down into the sewers after the minotaur gets blocked by the wall. Use the Key subweapon on the locked door at end of the sewer. This will trigger an intermission sequence showing Maria joining Richter. She can now be selected as a playable character at the main menu.
Original Game

Using Maria in stage two, fall down the first pit when the behemoth is chasing you and beat the rest of the level. You will then be taken to a secret stage three. Eventually, you will come to a point that has two paths. Take the upper path and continue along. You will soon come to a point that has platforms on the top and bottom. Break the vine at the top and continue to jump on the top platforms. Break the tombstone at the end and you can play the original version of Rondo of Blood!
Castlevania: Symphony Of The Night

When the minotaur chases Richter in Stage 2, immediately fall into the first hole to enter the sewers. Complete the level as usual by defeating the Boss. You will then start Stage 3', the alternate version of Stage 3. Every time your path splits in Stage 3', always select the upper route. Collect the Axe subweapon when it becomes available. Use it to cut the vine-covered cage near the skeletal snake that is attached to a wall. Jump on the cage to allow Richter to jump to the upper ledges. Continue along the upper ledges until reach a breakable tombstone that contains the item that unlocks Castlevania: Symphony Of The Night as a playable game at the main menu.