The concept of the Castlevania series can easily be repurposed for a Dungeons & Dragons campaign, especially as both franchises take a lot of inspiration from the same sources. The Castlevania series already has an unofficial link to D&D, as the box art for Castlevania 2: Simon's Quest is heavily inspired by the original Strahd adventure for D&D, as was the case with many NES games of the era outright copying movie posters and book covers.
The Castlevania series consists of horror action games. The story takes place in the same world as Bram Stoker's Dracula, except that Dracula keeps coming back from the dead. Dracula's Castle keeps appearing in the world and it's up to the Belmont family (or one of their allies, like Alucard) to brave its halls and defeat Dracula. This involves fighting through waves of bad guys and surviving numerous environmental hazards. The Castlevania series also provided one half of the Metroidvania name, thanks to the design of the incredible Castlevania: Symphony of the Night.
There is already a gothic-horror D&D 5e campaign, Curse of Strahd. The problem with Curse of Strahd is that it's designed in a nonlinear way, with elements that are meant to be decided randomly. This means that inexperienced DMs might have trouble running it. A Castlevania-themed campaign can be a lot more straightforward while maintaining a lot of the same iconic elements that new players will instantly be familiar with.
Curse of Strahd takes place in the fantasy realm of Barovia, which is one of the Domains of Dread from the Ravenloft campaign setting. The Domains of Dread can be entered at various points in the D&D multiverse, which means that it's easy to use any character type in the campaign. It's feasible for a Curse of Strahd party to consist of a cannibal halfling from Athas, a pious warforged from Eberron, a swashbuckling Giff from Spelljammer, and an academic Owlin from Strixhaven.
The Castlevania series is set in a fictionalized version of the real world. This means that humans exist, but the common D&D fantasy races do not. It wouldn't make sense for races like the dragonborn or tieflings to be hanging out in Europe in the Middle Ages, so the race options available to the players would need to be restricted. There are some options from Van Richten's Guide to Ravenloft that could be adapted for a Castlevania setting, as it features rules for Dhampir and Frankenstein's monster characters in D&D, the former of which can be used for half-vampires, like Alucard.
The players won't be as restricted when it comes to classes. The barbarian, fighter, monk, ranger, and rogue are easy to use, especially if the magical subclasses (like the Eldritch Knight) are restricted. Magic does exist in the Castlevania universe and there have been spellcasters among the playable characters, such as Sypha from Castlevania 3: Dracula's Curse and Charlotte from Castlevania: Portrait of Ruin. There are examples of warlocks and witches in the Castlevania universe and the D&D spellcaster classes can easily be tailored to fit them. If there is a Belmont among the party, then they would be a perfect fit for a paladin. The Vampire Killer whip can be adapted for D&D as a weapon that gradually grows stronger over the course of the story, with it gaining pluses and special abilities as the Belmont defeats monsters and becomes more experienced.
The foes in the Castlevania series come straight out of the Universal and Hammer movies, but the games have included a lot of creatures from mythology, as well as monsters from religious texts. Fortunately, most of these enemies have been statted out in D&D and can be found in the Monster Manual. As such, the animated objects, banshee, chimera, cyclops, succubus, gargoyle, genie, ghost, golem, griffon, hag, harpy, hippogriff, hydra, lizardfolk, lycanthropes, manticore, medusa, minotaur, mummy, revenant, skeleton, specter, sphinx, sprite, skeleton, vampire spawn, wight, will-o-wisp, wraith, wyvern, yeti, and zombie can all easily be slotted into the campaign. All of the animals from Appendix A can also fit, as would many of the nonplayer characters from Appendix B.
As for Dracula himself, the vampire in the D&D Monster Manual can be used as the basis for the character, with Dracula's Castle acting as his lair. The vampire is a CR 13 creature, but it also has a ton of weaknesses. This means a lower-level party could take on the vampire and win, especially if they have magic items that can deal with a vampire, like how the Sunsword in Curse of Strahd makes the final battle a lot easier.
The plot structure of a Castlevania D&D campaign is easy to work out, as the video games have used a tried and true formula for decades now. The campaign is set sometime in the Middle Ages in a version of the real world where magic and monsters exist, but both are so rare that they are thought to be myths. One day, Dracula's Castle appears, and its gates open, unleashing a horde of monsters on nearby villages and towns. Luckily, this day has been prepared for, as the Belmont family has been battling Dracula for centuries and they recognize the signs of his return. The latest warrior of the Belmont clan gathers a group of fearless allies and they set out to infiltrate Dracula's Castle and slay him.
The party background & motivations are easy enough to work out for a Castlevania campaign. A Belmont character will want to see Dracula slain, as will any of his loyal allies. Dracula has caused enough carnage over the years that there will be others seeking vengeance against him. There is also the promise of knowledge and treasure, as Dracula's Castle is filled with ancient tomes and the gold taken as plunder from his victims.
The campaign itself can start outside of Dracula's Castle, as the players can gain experience and story details from defeating Dracula's minions that are invading the countryside. The story will eventually bring them to the castle, which is where the bulk of the campaign can take place. It's possible to create a Metroid-style D&D campaign and go for the full Metroidvania experience, with the players exploring a massive map that is gradually unlocked by finding new upgrades, but the DM doesn't necessarily need to do all of that work.
The halls of Dracula's Castle can be ever-changing, with the layout shifting to create new dungeons. The bosses of these dungeons are bound to the castle through magic, and when the players defeat them, they gain more control over their surroundings, eventually giving them access to the big man himself. The Castlevania Dungeons & Dragons campaign can act as a simpler version of Curse of Strahd, with familiar concepts and monsters that will be useful for new players as they learn the rules of the game.
