What is Castlevania? At its heart, its the story of one man and his adventure to destroy Dracula. Stripped of all the story and additional play mechanics that the later entries added, the game boils down to a hero, a whip, and a very bad vampire that has to be destroyed, all presented with cinematic visuals and a killer soundtrack.
Released in 1987 for the Nintendo Entertainment System (1986 in Japan for the Nintendo Famicom Disc System, with a later Famicom cartridge release in 1993), Castlevania (Akumajo Dracula in Japan, which rough translated as "Demon Castle Dracula", "Devil Castle Dracula", or "the Satanic Castle of Dracula") was a very different game from other releases early in the "Silver Age of Gaming". It wasn't a run and gun shooter (like Contra) or an explorational adventure (The Legend of Zelda or Blaster Master), an RPG (Final Fantasy or Dragon Warrior) or a trippy surreal platformer (Super Mario Bros. or Milon's Secret Castle). What it did was take elements of a variety of genres and mix them together into something fast, sleek, difficult (but purposefully not "unfair"), and altogether new.
A platformer at its core, Castlevania gave the world Simon Belmont, heir to the Belmont clan and wielder of the "Vampire Killer", a whip capable of destroying the undead. Through seven stages, Simon had to run, jump, whip, and (most of all) survive everything the castle could throw at him. Unlike Mario, Simon couldn't touch any of the enemies without taking damage. Unlike Link, Simon could only collect a single additional weapon (sub-weapons, a sort of projectile, each with their own uses), and if he died, the weapon went away. Surviving Castlevania was about skill, persistence, and memorization.
As noted, it wasn't unfair (although it may have felt like it, the famous "Nintendo Hard"). As explained by original series director Hitoshi Akamatsu Castlevania was intended to be a "cinematic" experience, one that drew the players in and made them feel like they were actually playing as Simon. The goal was to create an experience that did require you to work for it but, with skill, players should be able to get through the game and appreciate the whole experience. To that end it had a strict set of rules and followed them; hit detection was tight, enemies had a clear attack paths and patterns, and once you understood the rules of the game you could predictably make your way through the adventure. Each obstacle could consistently be navigated, each challenge overcome, and in the end, the heroes would save the day.
When released, Castlevania presented Nintendo players with a cinematic experience, a blending of the Universal horror of old with action-twitch game play of the console. Everything was meant to put you into the experience of being in a Gothic horror film, and that visual style, blended with the audio tracks that helped make the game famous, set this title apart for some many other games on the console. It went on to be one of Konami's early hits, with a reported 800,000 units sold at the time (with those numbers jumping to 1.5 Mil after re-releases and additional ports).
So what is Castlevania? In the era of the Nintendo, there were Marios, and Zeldas, Fantasies (Final or otherwise) and Warriors. But for a time, in 1986, when the vampires came out, there was only one man to call, and one cinematic masterpiece to find him in. This is the game that launched a franchise, a title that can to define what could be done with the NES hardware, and an epic adventure that set its own genre (or two, eventually) and raised the bar for all future action adventure games to follow.