Honestly any gothic, horror or vampire themed anime or movie could be a source for CV. There are certain things that always set a vampire styled game or movie up so you are immediately in that surrounding. Darkness, castles, lonely paths through the woods, graveyards, ect
I don't see it personally for these two other than classic horror type settings.
I understand your sentiment, but have you looked closely at the pictures? The biggest giveaway is in the one where the assassins are crawling on the rooftops in
Cagliostro. Compare it to the background village of
CVIII's stage 1, and you see essentially the same angles and castle facade in both backgrounds.
Also,
Cagliostro has a sunken ancient Roman-style city hidden around it, and that could really explain where Castlevania got that idea from, which otherwise seems to be a non-sequitur. The structure of the clock tower and the bridge, and the maneuvering over the gears also seem very suspicious.
Finally, I have some bad news for you, perhaps.
Vampire Hunter D novels, manga, and anime movies likely established two big Castlevania tropes. A
whip-brandishing character against vampires in the first novel (1983) and first movie (1985, at least 9 months prior to Castlevania NES in Japan). And, SotN's long-haired, ethereally handsome and sword-wielding interpretation of Alucard is also likely taken from the titular character of Vampire Hunter D, who is also Dracula's anti-hero son with these same attributes.