Surprisingly, the changes between the Japanese and American versions were very minor. Even the Japanese game was entirely in English. The only major difference was in the intro - in the Japanese game, he is more feminine, a typical bishounen (pretty boy). He was hardened up to made less girly in the American release. The American release was also made a little bit more difficult.

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European/Japanese
However, the European release is a bit different...it's known as Castlevania : The New Generation (not like Star Trek) as some people don't like the word "blood" in the title of the game (the word "ninja" is also associated with violence and it kept out of titles as well.) The title screen has been changed to a sea of water (as opposed to blood) and Eric's spear doesn't impale him after death. Other small things like this have been censored (the zombies in the first level have changed color from purple to green, in order to lessen the gruesomeness.) From what I've been told, it's primarily Germany that does not allow these thing in video games and as a result, the rest of Europe suffers. Oddly enough, the difficulty level is the same as the Japanese version, and Eric's picture is unaltered as well.

American
European