I liked the second season lot more than the first season even if it was a bit more long that it should (lot of filler scenes to fill gore and time quotes without any real purpose).
Although I don't know if was a good castlevania adaptation, had too Little adventure for that, it was more like Game of Thrones wannabe with vampires. So may be wasn't a good adaptation, but it was certainly a good fantasy animation. The charácters had some really good interactions with each other and the action scenes were cool.
Carmilla appeared in some castlevania games, yes, but she is not particulary based in any of the portrays of her in the franchise, was more like a complete new Carmilla. Also, she appears in castlevania games, but Carmilla like Drácula's origin is actually on literature. She is the main charácter of a novel that outdates even Stocker's Drácula. And like Drácula with Vlad Tepes she is roughly based on an actual historical figure, a noble and serial killer called Elizabeth Bathory
Carmilla was the first "romantic" Vampire in fiction and is the second most famous vampire ever.