A sequel that isn't just more of the sameOne of the things I love most about the early 8-bit era was its approach to sequels. It wasn't just assumed, right off the bat, that a successful game's follow-up should play just like the first. As early as Donkey Kong Junior, we start to see a distinct lack of cynical assumptions about replicating the gameplay of the originator too rigidly. Zelda II: The Adventure of Link dumped a successful, overhead-view format for a controversial side-view approach. Teenage Mutant Ninja Turtles II: The Arcade Game adopted a much more enjoyable side-scrolling beat-'em-up format after the first game's alienating platforming and clumsy controls. Final Fantasy II, though it saw no North American release at the time, completely abandoned the concept of player-chosen character classes in favour of a wide-open and dynamic skill system. And of course, there's the infamous case of the English Super Mario Bros 2 -- which, while it was technically an adaptation of the unrelated Doki Doki Panic, was nonetheless how Nintendo chose to frame the next iteration of its flagship brand overseas.
I fully accept that Castlevania II is a fundamentally flawed experience beyond repair, but the raw ideas behind it were nothing if not daring. In this modern age, where we can have almost full knowledge of the nuts and bolts of a game before it's even released, and sequels are essentially expansion packs tacked onto their predecessors, it would be nice to have more surprises. In my mind, a good sequel is one that carries on the spirit and atmosphere of the original, but isn't content to merely copy its formula. Castlevania III returned to the straight-up, linear action game style, and it is a better game for it, but if we had three Castlevania games that all did the same old thing, each one would probably seem less special.
The dawn of the real-time day-and-night cycle
Castlevania II, as far as I'm aware, is one of the earliest examples of a real-time cycle of day and night in a video game. It wasn't a very complex system, but it was one of the game's more interesting features -- maybe even a good one! An unseen timer would tick away as you played, then suddenly a text box would pop up, alerting you that night time had arrived before fading to darkness.
There was, apparently, a sci-fi game on the ZX Spectrum in 1985 called Tau Ceti that implemented a day/night cycle. From what I gather, the position of the sun in the sky affected how shadows were displayed on buildings and ships, though it sounds like it had little impact on anything outside of visuals. Castlevania II still takes the prize for having a significant difference in game content between night and day.
A hazy glimpse into Castlevania's future
After Simon's Quest, Castlevania games return to a more linear style for a long time. RPG-inspired boosts in character power are outright avoided, and although some iterations like Castlevania III and Rondo of Blood have optional routes through stages, hidden levels, and multiple ways to progress through the game, that progress is always in one direction: forward. It isn't until the wildly successful Symphony of the Night on the Playstation that things open up again, and a new trend overtakes Castlevania's format completely.
I'm not saying Castlevania II: Simon's Quest is solely responsible for the modern style of the series, but to deny it had any influence would be short-sighted. I would be very surprised if the team who put together Symphony of the Night didn't at least say something like "Hey, why don't we think about making a more open game, like Castlevania II was?" Even though the second game might be regarded as a failure, it's reasonable to consider the developers shelving the ideas they originally were shooting for until the series was ready for them and better tech was available.