In the Inverted Castle, the player must unite 5 relics of Dracula in order to unlock the path to Dracula (Those being a Ring, Heart, Rib, Tooth and Eye of Vlad), a concept first utilized in Castlevania 2. All five are guarded by bosses based on the original Castlevania bosses.

In addition, every boss in the Inverted Castle save one are previous bosses and even playable characters in the series. Aside from the aforementioned five, the player meets Galamoth (Kid Dracula), Shaft (Rondo of Blood), the Doppleganger (Castlevania 3), three falsified heroes from Castlevania 3, with whom Alucard himself was a companion, and of course Dracula. The one exception is Beelzebub, whose first appearance was Symphony of the Night.

The optional boss battle at the top of the Inverted Castle, Galamoth, is a villain from the Kid Dracula series. Although his design is far more gothic and gritty to coincide with Symphony of the Night's style, he retains basic design similarities and even similar lightning based attacks.

There are two familiars that didn't make the cut during the transition to the Western market. These are the Half-Fairy and the Nose Demon. They are almost exact copies of the normal Fairy and Demon, but with different graphics. They are still programmed into the Western version, but their familiar cards are no longer available, and thus have to be activated using GameShark codes.

A debug room can be accessed using GameShark codes, which features some testing tiles, and a Japanese voice track, which when translated reads;
"Hi, I'm Joji Yanami. I'm not a young man anymore. I've done this job for a long time. Eyup. I've done a lot of stupid roles. Man, though, this year has been so hot, I could die. It's way too much... I'm an old man, you see. I can't do anything anymore. It's exhausting. Someone save me, please. It's already September. I think something will come of all this, maybe. Old age is old age. Maybe not, though. Yeah. Well, that's all. See you, then."

There are a large number of unused dialogue pieces in the game's data, including what may be an unused ending. Many of these sound files are alternate versions of voices that are already used in the game, but extended or slightly different.

A series of clips can be found in the game's sound files, which seem to be an unfinished ending. It seems to suggest that Maria intervenes in the fight between Alucard and Richter, where she becomes possessed or transformed by Shaft. It includes both winning and losing quotes, and ends with Alucard reflecting on the apparent death of both Richter and Maria. While the PSP and Saturn version allows a fight with Maria, it is not under these circumstances. There are no other suggestions towards this ending, but it is assumed to be an alternate ending during the fight with Richter when not using the glasses.

With the Lyric Card equipped, the player can activate a secret lullaby from the Fairy if Alucard sits in a chair for a period of time. This was only available in the Japanese version of the game, as the familiar that sings it was removed from the international release. But, Konami actually went out of their way to translate this song into English for the PSP version, too.
(Attachment will play both Japanese and English.)

The Saturn version includes a number of extra areas and features unavailable in the Playstation version. Most infamously being that of the Underground Garden, as the entrance is still visible in the Playstation version, though inaccessible. The player can glitch the game into allowing access to this area, but it is unfinished and only a glitched out save point is available.

When using the Rune Sword, the word "Verboten" will appear in its arc. This means "Forbidden" in German.

Dracula's castle is in fact Mont Saint Michel, in France.

If Alucard remains seated for a long period of time, he will fall asleep with visible "Z"s. If you have the Fairy familiar, she will also take a rest on Alucards shoulder, only to stumble when he gets up again afterwards. She will also do this if Alucard remains stationary for a prolonged period of time.

Every so often, when Alucard is turned to stone, there is a chance that he will become a giant stone gargoyle. This will make him invincible, but of course unable to do anything.

When inserting the CD into a CD player or PC and you attempt to listen to it, you'll hear a message from Alucard which says;
"As you can see, this is a PlayStation black disk. Cut #1 contains computer data, so please, don't play it. But you probably won't listen to me anyway, will you?"
You'll then be treated to a nice little bit of remixed music from the series.

At the start of the game, Death will take Alucard's equipment. If you continue to the Alchemy Laboratory, and then immediately return to the room you met Death, you will be faced with Slogra and Gaibon (the first boss fight). As soon as you attack either of them, Gaibon will grab Slogra and fly off, and you'll meet them again in their usual place.