Play as Maxim:

To play as Maxim enter your name as "MAXIM" at the name input screen.

Just like Richter's game in Symphony of the Night there is no story.  But Maxim is very fast, from the way he runs to the way he attacks.  You can attack about twice as fast as Juste.  Also he can not use subweapons, but he does have his shuriken.  In addition to that he has a triple jump and a jump that damage people (jump then up+jump in midair.)  Finally he has a super attack and can heal himself.

Harder game:

To have a harder game put your name in as "HARDGAME."

No spellbooks:

To have no spellbooks put your name in as "NO MAGIC."  Spellbooks are now disabled.

Boss Rush mode:

To unlock Boss Rush mode beat the game once.

Boss rush mode is where you fight all the bosses consecutively using no magic and one subweapon that you can pick only once.  Beating it on one difficulty unlocks the next one and so on.  The difficulty determines who your last boss is.  Easy goes to Minotaur, Medium goes to Legion (Saint), and Hard goes to Dracula.

Play as Maxim in Boss Rush Mode:

Go to the Boss Rush screen and hold L+R and keep them held down when you select a difficulty level..

Maxim has all his moves and plays just the same as in the regular game.

Unlock Sound Test:

To open up the sound test just get the best ending (beat Dracula.)

You can listen to all the songs here, obviously.  It has two songs in particular that aren't used in the game, Vampire Killer and the Castlevania 1 death song which are used in a special battle arena mode.

Play as Simon Belmont (Boss Rush Mode only):

At the Konami screen put in the old Konami code- Up, Up, Down, Down, Left, Right, Left, Right, B, and A.

This is the special boss rush mode.  Simon has his old NES sprite and plays just the same.  He can't switch direction in midair jump, no double jump or special moves.  Just walk, whip, jump, and use the Axe, Holy Water, or Cross.  He is a lot stronger than Juste, he does 138 damage with the whip instead of 97.  Vampire Killer plays for him and when he dies the CV1 death song plays.

Get the Worst Ending:

Of the three endings this is the worst one.  To get it just kill Maxim in Castle B (green map).  From what I can tell Maxim and Lydie both die, much like the SotN ending where Richter and Maria die.

Get the Medium Ending:

To get the semi good ending kill Maxim in Castle A (blue map).  You kill Maxim but Lydie survives, that's about all I know from the ending scene visually without being able to read the text.

Get the Best Ending:

This is the best ending.  To get this one fight Maxim in Castle B (green map) and equip Juste's ring (gold with red center, you start the game with it) and Maxim's Ring (gold with blue center, if you're at this point in the game then you have it.)  After fighting Maxim with them you fight Dracula.  Beat Dracula and you get the good ending.  From what you can see know that Maxim and Lydie both live.