A series of games starting on the NES and continuing to modern times, centering on Dracula and various people who try to stop him (primarily the Belmont family, but also including his half-human half-vampire son and some random kid from the future). The games on the NES and SNES were reasonably creative and very definitely NintendoHard platforming games, wherein the player had to wrestle with limited health and controls, as well as very agile monsters. These are the kinds of games that give LostLevels a run for its money in terms of difficulty.
Later on, with the advent of the PlayStation, a new CastleVania game style came to the fore, melding the old platforming gametype with RPG elements (to be fair, CastleVania 2 for the NES had some of this already). Now the player could do such things as: gain experience, find better equipment, cast spells, save the game, and become a completionist freak. Just look at the GameChallenge node for a bunch of Symphony of the Night challenges. Anyway, this new style of game has become the de facto standard, as all new CastleVania games have followed in its footsteps (well, save perhaps the PlayStationIi game, which I have not played). Castlevania 64, however, did not follow this pattern at all, and was more of a generic 3D action/platformer.
In general it's a series well worth checking out, if only for its formative effects on the gaming world as a whole. Much like LegendOfZelda in that sense.