It had been 100 years since Dracula's final defeat at the hands of Christopher Belmont (ignore Order of Shadows in this case), and the townspeople had grown more secure in their lands, happy in the relative peace of their world. All that, sadly, was about to change.
The followers of Dracula, performing a "Black Mass Ritual" (as seen in the official, in-continuity remake for the Sharp X68000 computer system), attempted to bring Dracula back from the grave. Their ritual, performed on the same day as the town's Easter services (presumably, that would be Easter day), used the magic of the day, perverting it for their dark purposes.
On that day, Dracula was resurrected, and his castle appeared in the Carpathian mountains, once again casting its dark shadow (not to be confused with the TV series) over the lands. In those dark times only one person could be called upon to go to the dark castle and defeat the demon Lord at the top: Simon Belmont, heir to the clan and bearer of the famed Vampire Killer.
Simon, entering the castle, waged holy war on the fiends and demons that roamed its dark and dusty halls. Battling past the worst beasts, such as Medusa, Frankenstein's monster, and the Grim Reaper, Simon fought to the top of the keep where he faced off with the Dark Lord Dracula.
As hero and villain battled, their war waged hard. The vampire revealed his true, demonic form, and Simon defeated the demonic entity, sealing the beast to the grave and releasing the castle's magic. Simon fled to a nearby overlook, able to watch as the castle crumbled into rock and dust. The land was released (for a time), and the villagers thought they would be able to live in peace again.
Castlevania was remade multiple times, for multiple systems. The two most noteworthy remakes though were Vampire Killer and Castlevania for the Sharp X68000 computer system (which was itself remade again for the Sony PlayStation under the name Castlevania Chronicles). Vampire Killer was noteworthy as it implemented an adventure style game mechanic where-by Simon had to explore the castle, searching for keys to unlock doors. The game play would go on to influence Castlevania II as well as being a precursor of the whole "Metroidvania" movement in modern Castlevania games.
Meanwhile, Castlevania for the X68000 (often referred to simply as Castlevania X68000 or Castlevania X68k) would introduce the concept of the "Black Mass Ritual", where-by Dracula's followers could resurrect the demon via a sacrifice. This helped to flesh out the story of the original game, which was rather threadbare (as many of the early NES games were).
Interestingly, the game was thought of as a parody of the classic Universal Studios monster movies from the 30s and 40s. Many of the monsters from the classics appear (not just Dracula, but also Frankie, and the Creature from the Black Lagoon -- in the guise of the lowly Mermen). Other games would feature more of the monsters, giving a wider connection to their forebears. Even upon defeating the game, the parody continued, with many of the monsters being "played" by humorously named actors, references to the actors from the original movies such as Frankie being played by "Boris Karloffice" and Death being played by "Belo Lugosi". Dracula was played by "Christoper Bee" who was a reference to Christopher Lee, the actor who played Dracula in the Hammer Studios film series (also classics).
The Lords of Shadow reboot continuity also features Simon Belmont, although this new Simon's adventure takes place 600 year prior to the main continuity adventures of Simon Belmont. Lords of Shadow Simon is actually the son of Trevor Belmont (with no love shown for intermediary, Christopher Belmont), not his several-times-over-great-grandson. Simon's adventure is very tightly plotted into the Lords of Shadow continuity, and isn't just a remake of the original Castlevania.