Hellfire (15MP): up, down, down-right, right and square
Summon Spirit (5MP): left, right, up, down and square
Soul Steal (50MP): left, right, down-right, down, down-left, left, right and square
Dark Metamorphosis (10MP): left, up-left, up, up-right, right and square
Tetra Spirits (20MP): hold up for 2 seconds, then press up-right, right, down-right, down and square
Wolf Charge (10MP, must have "skill of wolf", and must also be in Wolf form): down, down-right, right and square
Sword Brothers (30MP, must be using the "Sword" familiar for this to work): down, down-right, right, up-right, hold up for 2 seconds, then press down and square.
Wing Smash (8MP, must be in bat form when performing this): hold X, up, up-left, left, down-left, down, down-right, right and then release X 

Axe Lord Armor  -  After beating the game once, start a new game and enter your name as AXEARMOR. You'll find a cool new suit of armor in your inventory. 

Beat Doppleganger Easy  -  Okay, for players who have a hard time with Doppleganger (the Alucard look-alike), hit him so he lands in a corner, and repeatedly jump at him and attack. He will reliably jump into your blade repeatedly. This works on both the Light and Dark Castle Doppleganger. 
OR
Before you fight the doppleganger in the second castle, equip the Medusa Shield. Hit the doppleganger with the Medusa Shield, and he will turn to stone. Attack him mercilessly with your weapon until he turns back to flesh, then use the shield on him again. 

Beating Galamoth  -  Here is an easy way to kill Galamoth. First find the Alucard Shield and the Shield Rod. Equip them both before you take him on. As soon as he starts to stand up, activate the Shield Rod combo (Square+Circle). When you stop flashing, run toward Galamoth so only the Alucard Shield is touching his leg. Great deals of HP will be taken away from him. I tried this and ended up killing him in 8 seconds! 

Belmont's Special Moves 

Whip Uppercut: down, up and X
Run: right right, or left left (hold after second press)
Whip Twirl: hold Square, then tap any direction repeatedly
Slide: down and X
Slide Jump: down and X, X again immediately after the first one
Back Flip: X twice, quickly
Whip Dash: up, down, downright, right and square (Richter is invulnerable when performing this)
Whip Lunge: left, then right and square (or vice versa) 

Bonus Soundtrack  -  Put the Castlevania CD into any regular CD player. Play Track 2, and you'll hear a special song. 

Easy Familiar Experience  -  Ok, easy way, but a bit time consuming, to get experience for all your familiars. I believe you have to have it beaten once first. but I may be wrong.

As you start the game, go through like you normally would. But this time, don't but a lot from the librarian. Only get the Jewel of Open, and 1 or 2 Library Cards. The proceed with the game.

Once you get in the inverted castle, keep going and get AS MUCH MONEY AS POSSIBLE. Once you get 500,000 go back to the librarian, and purchase the Duplicator. Then go BACK to the inverted castle, and equip it. By this time, you should have at least ONE of the Power of Sire items. Equip that as well. Voila, you have an infinite supply now as long as you have the duplicator equipped.

Now go to the room in the inverted church thing (forgot the name off hand), and enter the room with the big sword that is surrounded by spears and shields. Use the Power of Sire, then leave the room, and go back in, use it again, and so on and so forth. Within an hour I had ALL my familiars powered up to 99/99. 

Effortless Experience Gain  -  To get effortless experience follow these simple steps: 

Play through the game like normal until you get your bat familiar.  At this point go back to near the start of the game where the zombies keep coming out of the ground. then all you do is stand on the step on the stairs where the zombies can't reach you and turn your bat familiar on. Just make sure that you are far enough away that the zombies won't hurt you if your familiar misses one of them. The bat familiar will kill most of the zombies that appear, also make sure you are looking in the direction of the zombies or the bat will not hit them. The best part about this trick is that the familiar will kill the zombies and both he and you will get the experience making this absolutely simple in gaining levels. In three hours my bat went from level 2 to level 60! 

Enhanced Luck  -  After beating the game once, start a new game and enter your name as X-X!V"Q. You'll start with 99 luck and the Lapis Lazuli item, but your strength and hit points will be reduced. 

Explore an Inverted Castle  -  If you defeat the possessed Richter Belmond in Dracula's Keep, you'll see the ending. But what you may not realise is that this is the 'bad end', and you've only seen half the game! To get to the real ending:

First, make sure you've gotten both rings from Maria. The first you receive after defeating the Succubus in the 'Nightmare' stage. The second is past the hall with all the spikes in the Cathedral (you need the SpikeBuster Armor from the Catacombs)

Equip both rings at once in the room with the big clock, and the floor will open. Down in this 'lair' Maria is waiting for you once again. She'll give you a vital piece of information.

Once all this is done, then go to Dracula's Keep and meet Richter. He'll attack you, but you will now be able to see a green orb floating above him. Attack the orb, but don't kill Richter (hint: the Sword Companion will serve you well here) After a bit of attacking, the orb will break open, revealing everyone's favorite loathsome 18th/19th century warlock: Shaft! (daaaarn right! right on, bruthaman!)

This also breaks Shaft's spell, freeing Richter from the spirit of Dracula. Alucard will tell Richter and Maria to escape while they can as the clouds swirl and open up, revealing a mirror-image of the Evil Castle! Shaft escapes into the Inverted Castle, and of course it's Alucard's duty to chase after him!

But just when you think you know the layout already, you find some crucial elements of the castle have been rearranged, it's stocked with bigger and badder enemies, and even the music has changed! 

Extra Options  -  At the name select screen, enter your name as AXELORD then go to the Settings screen. 

Fiddle with the Logo  -  When the words NOW LOADING are on screen, you can manipulate them with the d-pad. 

Get the Holy Glasses and all Four Endings  -  To get the Holy Glasses you need the gold ring (after beating the Succubus in Nightmare, located in a side path in the long tunnel to the underground) and the silver ring (in the hall of spikes in the tower, you need the Spike Buster armor from the underground.) Take these to the clock tower and equip them. You'll get a L-O-N-G sequence before it lets you go down. Follow the path to find Maria, she'll give you the Holy Glasses.

Endings:-

1. Simply go to the castle tower and defeat Richter without meeting Maria. Alucard will escape the destroyed castle and say a few things before going back to his eternal sleep.
2. Get the Holy Glasses and defeat Richter. Maria will have also escaped and tell you she's continuing her search for what happened to Richter.
For the 3rd and 4th endings, do this first: Get the Holy Glasses and equip them, but DON'T kill Richter. Instead, attack the green orb above him until it shatters. From here you'll go to the inverted castle.
3. Defeat Dracula with anything below 200% and you'll see Maria and Richter waiting for you outside the castle. Maria will decide not to follow Alucard and will return with Richter. 
4. Get AT LEAST 199.1% and defeat Dracula. The others will be waiting, but Maria will decide to follow Alucard and try to help him cope with what he is. 

Heaven Sword Special Attack  -  You can get Heaven Swords as a rare item by killing the Cloaked knights (flying knights that toss their swords at you). If you have two of these, equip one in each hand and press both square and circle to activate the special attack. 

Librarian's Secret Stash  -  To get the librarian's secret stash under his chair you must have the super jump relic. After you have obtained this item go to the room under his. There will be a narrow vertical space that (when you jump into it) will lead to a Life Max Up.

Now jump repeatedly without touching the floor, but make sure the librarian's chair goes back on the floor. If this is done correctly you will first get the Ring of Arcana. If you continue to do this you will then obtain the Dracula Tunic and after that the Axelord Armor will follow. 

Music Select  -  To gain access to the excellent soundtracks, complete the game (fully, with over 190%) with Alucard and restart a brand new game with Alucard (do not cover over the old one). When you get to the Librarian, simply walk up to him and a music select option will appear with the 'buy/sell' box. 

Play as Richter in the Japanese Version  -  After beating the game once, start a new game and enter your name as RICHTER. You can play now as Richter Belmont. 

Secret Rooms  -  Here are the locations of some of the secret rooms in the game. 

Room 1: Outer Wall
When you enter the stage from the Marble Garden, go down to meet an Armor Lord. Beat him and go to the left. Hack the wall on the left wall to get a Pot Roast. Stand in the Space for 20 seconds and you will go into a room with items 

Room 2: Alchemy Lab And Necromancy Lab
In the Alchemy Lab, hack the wall in the first room going up. Inside is a Heart Max-up. The one in the Necromancy lab of the inverted castle is in the same spot. 

Room 3: Colossem and Reverse Colossem
In the room with Blade Soldiers and Blade Masters, go to the first pillar, face left and use a few axes to reveal a passage. 

Rooms 4 and 5: Orlox's Quarters and Akmodan's Chamber
The first room is in the room with the Sprit Sword. Go to the right to find a dead end, hack the wall to get some good items.

The second one is later in the stage. When you are in courtyard with the Vahalla Knights, go down to the ground and proceed left. Proceed left until you are in a room with the bridge. Hack Every part of the wall until you reveal the room. Do the same in Akmodan's Chamber's (Orlox's Room in the Inverted Castle) 

Room 6: Clock Tower and Reverse Clock Tower
In the room before the Birdman boss, Hack the right wall at the top of the room. 

Room 7: Pit to the Catacombs
Use the Demon Familiar to flip the switch after the Cerberas Boss to reveal a room. Do the same in the inverted castle. 

The Shield Rod Trick  -  If you have the Shield Rod you can do this trick. Equip the rod and any shield. Then, all you have to do is hit the left hand button and the right hand button (normally the O and Square buttons) simultaneously and you will do a special move that has to do with the shield you have equipped. Every shield does something different. Experiment with different shields to find out which one you like. Also, when you are equipped with the shield rod if you press back, forward, then attack Alucard launches a star from the rod. It must do something besides look really cool, but I haven't messed around with it that much to find out. 

True Hellfire  -  There is an interesting effect if you cast the Hellfire spell, (up, down, down-right, right and square) then press and hold UP, AFTER the teleport, but BEFORE the three fireballs go off. It shoots TWO of the black fireballs demonstrated by Dracula in the opening scene. As far as I can tell, it does no more damage, and costs the same, but it DOES punch through targets rather than be absorbed.