Almost all of the games work like this: your main weapon is a whip, that can be upgraded at least twice. In most versions, you can only whip left or right, although in the newer games you can whip vertically as well. In the earlier games you had 16 bars of life, although you now get 20 in the newer games. You collect hearts that can be used to buy stuff (in Vampire Killer and Simon's Quest) and are also used for the special weapons (dagger, axe, boomerang, holy water and stopwatch.) These things have been standard in almost all games, like the fact that your characters jumping animation frame is exactly the same as his ducking frame.
One problem I have with the whole series is the inability to get all of the best features into one game. If you were to take the nonlinearity and characters of Castlevania 3, add in the graphics and play control of Castlevania 4, and mix the varied difficulty settings of Bloodlines, and still retain some of the RPG elements of Simon's Quest, you'd have the perfect Castlevania game. Yet that hasn't happened. We can only hope with the new Castlevania game coming for the Saturn and Playstation, The Bloodletting.
There are a few interesting tidbits regarding the Castlevania games. I have heard that it is based off of a Japanese anime called Vampire Hunter D. If anyone has any more information (or better yet, pictures) send them this way. Another interesting thing is that Simon Belmont starred along Kid Icarus, Mega Man, and Captain N in the NBC Saturday morning cartoon series, Captain N and the Game Masters (I think.) All I can remember is that he looked like an idiot, and talked like a snob. There was one episode involving the Castlevania games, where the team met a skateboarding Alucard. Again, if anyone has any pictures of these, I'd be incredibley grateful!