A funny thing happened between 1987 and 1988: a bit of Metroid got mixed up in the Castlevania series. Now, we're not talking about Simon Belmont going off the fight Mother Brain (although we at the Inverted Dungeon think that would be an epic adventure worthy of a proper release). No, what happened was that Simon Belmont was freed of the confines of Dracula's Castle was given the countryside of Romania to explore.
As it goes, Dracula was dead, defeated by Simon, but he had his bits scattered across Romania. A curse was placed on Simon that would kill the Belmont if Draula's bits and pieces weren't put back together (ensuring Dracula's resurrection). To find Dracula's body parts, Simon had to adventure through a variety of locales -- forests, caverns, graveyards, and mansions. Along the way he could upgrade his equipment (multiple whips and daggers, was well as a variety of other equipment), find important items, and even level up.
In comparison to Castlevania, Castlevania II was a different, more diverse, more fleshed-out experience. Gone was the linear path through a castle, replaced with mysteries to solve, ruins to explore, townspeople to talk to, and quest items to find. This was a different kind of game.
Of course, after Castlevania II, the series went back its more traditional roots -- heroes in leather kilts on a more-or-less linear quest through Dracula's castle. For a long while, Castlevania II seemed like the odd duck of the series. And then, thanks to Castlevania: Symphony of the Night (and the rise of the Metroidvania), Castlevania II seems more like an early precursor to where the series would eventually end up, and not just this strange one-off (or two-off, counting Vampire Killer).