Castlevania was a great game, and Konami did well in bringing something new to the table for the following games. The NES trilogy is a great series and the third game is the perfect final chapter for the 8-bit era. In fact, the fourth game on the Super NES was actually a little bit of a disappointment because it did not continue something established in the third game.
Castlevania II was a bold change of direction. While the original game was a straight action platformer, the sequel focused on exploration, finding hidden secrets, visiting towns and upgrading equipment. It was significantly easier than the first game. The Grim Reaper was not much of a challenge after being incredibly difficult in the first game, and Dracula is the easiest final boss I can remember fighting in a video game. If you have holy water, keep dousing him with it and you will paralyze him and win without taking a single hit.
Where Castlevania II was groundbreaking is the fact that it had alternate endings. You had three different alternate endings depending on what you do in the game. That does not seem special now, but thirty years ago it was amazing. It was a motivation to play through again to see the other endings.
Castlevania III upped the ante. It was a huge game with alternate paths, so you had to play through the game three times to play all of the levels. Where it really shined is introducing alternate characters: A thief, a mage and a good vampire. (That was Dracula’s son Alucard, a star of later games.) Getting to play as other characters was really cool. Unfortunately, Alucard was a huge disappointment, because he was so weak in combat. He should have been the most powerful of the four characters in the game.
Castlevania IV was an improvement over the NES trilogy in almost every way. Being on a 16-bit system, it had drastically improved graphics and sound. The player could whip in eight directions as opposed to only two directions in the NES games, and the whip went limp if you held the button down after a strike. Flailing the limp whip was the only way to defeat certain small enemies. My primary complaint was that the ability to play as other characters, which made part III so special, was absent in part IV.
Overall, the games continued to make strides and improvements. One could argue that I, III and IV were the true trilogy as they were the most similar in structure and game play.
Things would not go so well for Castlevania on the Nintendo 64.