You could feel Warren Ellis's contempt for the source material radiating off the script. The way he cut Grant just to dunk on him for so long, it probably ate more voice acting money and screen time than having him in it. Then making up Japanese characters to go "See how much they suck?" and propping them up as metaphors in front of the titular castle. He turned Isaac into his OC, and he might have genuinely been the best character. But you could see everybody else becoming dumber to make that possible, to the point where even Isaac wonders why people act suicidally stupid just to give him more action scenes and dialogue.
There's no way to make a show out of Castlevania without changing a lot of it. And there are melodramatic anime cliches I'm glad they avoided. I don't think faithfulness to the source material matters next to telling a good story. But the sheer self-indulgence happening gave me a taste for referential slop. The animators struggling against the budget to show off stuff from the games carried the show.