The Castlevania Adventure
For a time, Americans didn't have a clue about what was going on with the Castlevania continuity. Everyone could understand Castlevania and Castlevania II, as one was a direct follow-up to the other. But then the Castlevania Adventure came out (Dracula Densetsu in Japan, translated as "The Legend of Dracula"), and it was either a direct sequel to the two previous games, or a distant sequel, or a distant prequel. Bad translations from the Japanese instruction booklets, as well as the normal complete fabrication of story in the American booklets, had this game being another adventure for Simon Belmont. From a certain standpoint this made sense as Simon was the hero of the series so why would audiences expect there to be another member of the clan fighting vampires?
Of course, this was only confused further when Adventure received a direct sequel, Castlevania II: Belmont's Revenge, with a story describing the adventure of Christopher Belmont. So where did it fit into the series chronology, and how? What is the true place of The Castlevania Adventure within the scope of the series?
In Japan the answer was clear: this game was attributed to Simon's ancestor, Christopher, and took place 100 years before the other games... although, then they got Castlevania III, which also said it took place 100 years before hand and stared Ralph Belmont (Trevor to his American fans); it took a while for Konami to clear all that up. That said, on its own merits it doesn't really matter if The Castlevania Adventure is a sequel, prequel, or even a tiny-sized remake -- the game is about a Belmont going to Dracula's castle to defeat the evil within. Some would argue you don't really need more story than that.
On it's own merits, The Castlevania Adventure is a lesser entry in the Castlevania saga. Developed by a third-party studio (Konami has been tight-lipped about staffing and development on their early titles, but it's likely this was an outsourced project as the main Castlevania was working in the third entry, Castlevania III), the game did manage to take the very basic Castlevania game play and port it to the underpowered Game Boy system. In the process, though, it lost a fair bit of the magic of the series in the process -- the game was slower, lacked the standard sub-weapons that made the combat interesting, and was brutally difficult, lacking the balance that helped make the original game so playable. The adventure itself was scaled down in size as well, featuring only four stages when even the original game had seven. Essentially, while the game looked like Castlevania it didn't really feel like it.
That said, the game did sell well at the time, racking up a reported 2.5 Million units. This made it strong enough to dethrone even the original game as the best seller in the series at the time (assuming the sales numbers we have are accurate). The Game Boy was hot hardware and The Castlevania Adventure was released at just the right time to capitalize on it. Where Konami was less than happy with the sales numbers of Castlevania II (itself an odd-ball sequel that didn't sell as well as the original), this game gave Konami another jewel for what was becoming one of their flagship series.
And, in a way, without The Castlevania Adventure we wouldn't have gotten its absolutely sublime sequel, Castlevania II: Belmont's Revenge. Adventure, then, graced us with our first non-Simon hero and the portable titles only improved from here. While it may not be much fun to play in the current era, the game does function as an important milestone in Castlevania history and helped prove to Konami that there was life in this series.