Video games usually tend to get better with sequels (unlike movies.) Castlevania III is no exception and is by far the best of the 8-bit series. The genre returns to the action of the first game with a little bit of path choosing taken from the second.
The story takes place 100 years before the first with Simon's ancestor Trevor Belomont in his bid to destroy Dracula. Trevor uses the traditional Belmont whip, boomerang cross, holy water, dagger, and axe. This was the first game in the series to feature unlockable characters. Your three sidekicks include Grant Dynasty, a pirate that can climb on walls and ceilings, Sypha Belmundo, a sorceress that casts elemental spells, and Dracula's own son Alucard who can turn into a bat, and is also the star of Symphony of the Night on the PS1.
Unlike subsequent games you can switch between your chosen sidekick at will, and what character you choose determines the paths you can take in the game. Completing the game with Grant is the easiest option while Alucard adds a lot to the challenge.
The game entails Trevor's journey from the outlying villages into the Castle to kill Dracula. Some of the early levels can get a little repetitive (as in killing wave after wave of floating jellyfish things in the Transylvanian countryside) but once you actually get to the castle it's pure Castlevainia. Other levels include the standard Clock tower with resident annoying medusa heads, a ghost Pirate ship, underground caverns, stone gardens with statues that turn into massive cyclops monsters, etc. Each level does a good job as far as design goes without much in the way of "dead space," i.e. areas where you really do nothing but walk. But they tend to look a little alike as they all use common sprites (a beige-tan square brick being the most common building block.)
Where this game shines is in the assortment of enemy types and bosses. All the classic Castlevainia monsters are there, ghosts, skeletons, those annoying bone towers, etc. and they all look good. Each enemy has a unique attack pattern. Although 8-bit A. I. usually consists of just walking into you a few of the bad guys here actually try to hit you with weapons once in a while. Boss types are among the best in any 8-bit game. From the get go you are presented with hulking hammer wielding cyclops monsters, twin dragons (which would make an appearance in many more Castlevania games,) Medusa, Frankenstein with annoying Igor, Huge winged demons, etc. Many of the bosses scream a (somewhat muffled) death cry as they burn up when killed. This was 1989, any voice in a video game was impressive.
Dracula himself attacks with the usual teleport and fireball technique, then morphs into a huge demon that takes up the whole screen. You have to leap across different platforms and pepper his head with attacks whilst simultaneously dodging laser beams. Pure genius. He also has a very clear (and very evil sounding) laugh that will give you nightmares.
Unlike Castlevania II everything here seems a lot more polished. Each character has strengths and weaknesses and their own technique to play. Thankfully they also have a good sense of "weight," as in no more weird floaty jumps. All this is good because the difficulty ramps up a lot towards the end (the usual Castlevania four hits and you're dead applies.) Even veteran players are going to get good use out of that password feature.
Probably the best action/platform/adventure game on the NES, period. Any fans of Symphony of the Night or the recent PS2 or GBA Castlevania games owe it to themselves to track a copy of this game down.