Sound Test

Go the the password menu and enter a password of all hearts. This will take you to a sound select menu, where you can listen to most of the game's music, although a few tracks (such as the intro theme) are missing. Interestingly, hackers have found that the castle select screen music was originally on this sound test, but has been dummied out. In the Japanese version, this theme is called "New Church" whilst in the American version it was changed to "Select BGM". It seems the "New Church" name wouldn't stick, as it's known as "Journey to Chaos" on Castlevania soundtracks.

Extra lives

Enter a password of candle, candle, heart, heart. This will take you to the castle select screen. Select a castle and you will start the game with nine lives instead of two.

Hard mode

Enter a password of blank, orb, blank, orb. This will take you to the castle select screen, but now hard mode will be turned on. From what I can tell the main difference with hard mode is that more enemies can degrade the power of you whip. In normal mode, getting hit by a vampire bat or raven won't degrade your whip's power, but in hard mode, it will.

Hidden items

In nearly every stage there are items hidden in walls, which can be revealed by whipping them. If you have a full life gauge, meat items will be replaced with large hearts.

Crystal Castle

Rock Castle

Plant Castle

Cloud Castle

Dracula's Castle, part 1

Dracula's Castle, part 1

Secret rooms

Most of the stages also have a secret room to discover, by climbing certain ropes and moving through the ceiling. This will lead you to a new room which will have some useful items in it.

Crystal Castle

Rock Castle

Not exactly a secret room, but a shortcut: by jumping at the above wall you'll go through it, allowing you to move across and skip the couple of rooms that are down below.

Plant Castle

Cloud Castle

Dracula's Castle, part 1

Dracula's Castle, part 2