The granddaddy of the franchise, Castlevania was a top selling game by the Konami company released for the original NES.
Story
Welcome to the Hell House. You are Simon Belmont, part of a famous line of vampire slayers. It is your task to enter Dracula's Home of Castlevania in order to defeat the dark lord and his minions. You are accompanied only by your trusty whip, dubbed Vampire Killer, and your hatred for the forces of evil. As an interesting aside, in the games credits, Simon Belmont is listed as Simon Belmondo, and all the bosses are a parody of past horror movie performers.
Gameplay
Castlevania is a side scrolling action platformer. You control Simon, and can move in basic directions, jump, and attack with a whip and special items. Your whip is upgradeable to three levels, extending and doing more damage with the upgrades. Special items include holy water, a knife, an axe, a stopwatch, and a boomerang, and use hearts, which are gathered by smashing candles spread throughout the stages. You can also upgrade your special weapon to throw two or three at a time.
There are six stages in Castlevania, which are divided into large areas separated by doors. If you die, the doors act as a continue point, but if you lose all your lives, you must continue from the beginning of the stage. There's a map in between stages that shows your progress through Dracula's castle. The game is fairly easy until around the fourth stage, where it starts to become daunting.
Each stage has numerous smaller enemies and features a boss, modeled after familiar ghouls. The bosses include a Giant Bat, Medusa, Mummies, The Creature, Death, and finally the dark lord Dracula. While the game is tough enough for many, once you beat it, you can take on a harder version featuring more and faster enemies.
Graphics
The graphics of this game are mostly dark, as you would expect inside a nightmarish castle. The enemies are distinct and vary in size, and backgrounds are pretty to look at and help immerse the gamer in this surreal nightmare. One exceptional graphic is the moon shining on Dracula's tower ominously. The different areas in the game are all well defined, although the layout can get pretty familiar quickly. The Count really looks suave, until he transforms into a horrible bat monster.
Sound
Castlevania has terrific music. Its main theme has been reused in many later sequels. Most of the music is very melodic, usually faster paced stuff that's a cross between classical, metal, and the distinctive sound of the NES synthesizer. There are lots of great tunes in this game. The sound effects are also great, from Simon Belmont's grunt when getting hit to the shattering of the bottles of holy water. There are a bunch of great sound effects throughout the game that really enhance the overall experience.
Tips and Tricks
There are hidden treasures spread throughout the game that can be found by waiting in the right spot, jumping, or whipping certain areas. A guide for these can be found online.
Holy water is one of the most effective weapons in the game to fight bosses after the first level, as it can freeze many of them in place while inflicting multiple hits.
Special weapons can be upgraded by killing 10 enemies in a row with a special weapon and then whipping a candle, up to 3 weapons on screen at a time.
Overall
I would give Castlevania a 4/5. One of the things it lacks is no saving or password system, which is frustrating when you have to start from scratch each time. The game also only has six levels, which leaves it a bit short, but the levels are fairly long. The sound is still excellent even after many years, but the graphics show the early limitations of the NES. I really like the side-scrolling aspect of the game, as recent Castlevania versions have become pretty repetitive Metroid clones. Actually defeating the game can be a huge challenge, but I appreciate the difficulty of older NES games. There's a ton of sequels to choose from, including Castlevania II and III for the NES.