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.
Released in 1987 for the Nintendo Entertainment System (1986 in Japan for the Nintendo Famicom), Castlevania 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, 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, presistence, and memorization.
But it wasn't unfair (although it may have felt like it). Castlevania was a hard game, but at is core, it had a strict set of rules and followed them. Hit detection was tight, enemies had a clear attack path (and sometimes patterns). With the right know how, each obstacle could consistently be navigated. In the end, the best of the best made it through, but anyone could practice enough to get to that upper echelon.
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 game to find him in.