When looking a the Castlevania series, there is a clear point where the old titles (old-school, platform-heavy action games) gave way to the Metroidvania games of the modern era: Symphony of the Night. That game introduced exploration and adventure elements into the mix, taking the series into an entirely different direction.
Except, what if Symphony wasn't the watershed moment it's credited as. Sure, Symphony is a fantastic game, and all the elements that made up the game were near-perfectly balanced. But it wasn't the first to make Castlevania into an adventure game. Before all the sequels, there was Vampire Killer.
Released one month after the original Castlevania (and never released in the United States), Vampire Killer used the same elements (Simon Belmont, Dracula, the whip, the sub-weapons), but featured remixed levels, more items, and (most importantly) game-play that required exploration over run-and-whip action (not that you weren't constantly assaulted by enemies). In the game, Simon has to explore every nook and cranny of each stage looking for keys -- keys that will open treasure chests and skeleton keys that will grant him entrance to new areas of the castle. Each stage was comprised of four areas, plus a boss room, and each area was a series of interconnected, full explorable screens. As the player, you had to memorize the various locations for the keys to progress further, and each area had to be unlocked in kind to reach the final goal.
The game, though, wasn't as well balanced as later entries. On top of the exploration and already difficult gamplay (ported from the original game), enemies moved more unpredictably, keys could be hidden is difficult (or impossible) to get to areas, and there were no continues -- three lives and you're back to the start. These elements combined to make one of the more frustrating experiences in the Castlevania series.
On its own, Vampire Killer is a flawed game with some really interesting ideas -- ideas that would go on to directly influence the first sequel to Castlevania, Castlevania II: Simon's Quest (in theory, anyway -- no one from Konami has ever confirmed this).