SOCIAL
Forgemasters Forum: A forum for Castlevania game fans. Open to new users!
Vampire Hunters Union: Castlevania games Discord server run by yours truly. Est. 2020 and still going strong!
Vampire Hunters Union Webring: Webring borne from the Discord server made and managed by my friend Noa! You don't need to be in the Discord server to join the webring, or vice versa, if you don't want to.
MAIN CANON
Castlevania cutscenes playlist, in timeline order - For the emulation-challenged. NOTE: some endings are not currently available in any youtube videos. I have compiled the most complete versions to the best of my ability.
Symphony of the Night prequel comic - A short prequel manga illustrated by Ayami Kojima and included in the Japanese version of the game. Translated into English.
Castlevania: Curse of Darkness prequel manga - Cancelled after two issues, which have been scanned, translated into English, and archived here.
OFFICIALLY LICENSED NON-CANON MEDIA
4Koma Translations - Translations of the various 4Koma comics made for Castlevania games.
Akumajou Dracula: Kabuchi no Tsuisoukyoku English translation by Shiroi Koumori - A novella sequel to Dawn of Sorrow.
Grimoire of Souls (beta) - Cutscene archive.
Castlevania: Nocturne of Recollection - Radio drama non-canon sequel to Symphony of the Night. English subtitles provided.
INFORMATIONAL
Mr. Perfect's Castlevania Realm. Games directory. A beautiful, old-web-style site that's still active today. There's tons of goodies on there, but this link directs straight to the directory for all the games. Click on any game from there to find scans of packaging, game manuals, etc. in every regional release.
Castlevania Dungeon user Nagumo's compilation of Castlevania dev interviews: Some are translated, some are not.
Shmuplation's Archive: Proceed and search up 'Castlevania' for a treasure trove of translated developer interviews, not archived in the above link.
Vampire Killer: A resource with both translated canon information from game manuals and guides, plus scans of Ayami Kojima's art - including the entirety of her artbook, Santa Lilio Sangre.
Antifandom's mirror of the Castlevania Wiki: Fandom is inundated with ads and recommendations for other wikis to the point where it's borderline unusable on its own. The information on this wiki is not necessarily reliable, but it's usually somewhat believable. Keep in mind the EGM fax cited on some pages is retconned out of canon, and the Trivia sections are a particular mess. Game data tends to be hit or miss.