When viewing the whole of the Castlevania series, the temptation is to say there are clearly delineated eras for the franchise, from the early action-platform games to the Metroidvania later titles (to say nothing of the much later Lords of Shadow sub-series). But while would be easy to use Castlevania: Symphony of the Night as the clear break point between old and new, that's simply not the case.
While it is true that Symphony is the game that perfected the Metroidvania style, it wasn't the smash hit, initially, that Konami wanted or expected. Thus, instead of immediately putting eight more Symphony-clones into production, the company had other internal groups craft their own Castlevania experiences. Released a short 8 months later in Japan, and a few months later in the U.S., Castlevania: Legends (Akumajo Dracula: Dark Night Prelude in Japan) is something of a step back for the series, a Game Boy game beholden to the very old-school platforming of old with none of the Symphony charm worked in.
Considering the release dates, this game was likely developed concurrently with Symphony of the Night (and it's possible this team saw what Koji Igarashi was doing on Symphony and that's the reason Alucard is included here as well), a game put into production for hand-held systems that would hopefully capitalize on the expected success of the PlayStation title, leading to greater sales for both. Unfortunately, two things hobbled this title: one, Symphony of the Night was not a break-out smash hit in Japan and, two, this game is much more beholden to the old Game Boy titles (like The Castlevania Adventure) than anything Symphony had on offer.
Set up to depict the earliest Belmont encounter with Dracula, Castlevania: Legends follows young Sonia Belmont, progenitor of the clan and the first female carrier of the famed Vampire Killer whip in series history. On her journey, Sonia would meet up with Alucard, getting advice from the Dark Lord's son to aid her in her quest. She'd then end up collecting special items and taking on Dracula, leading to that vampire's first known defeat (at least until Castlevania: Lament of Innocence removed this game from continuity).
It's just a pity that Sonia's game wasn't very good. While Legends was an admirable effort to explain the origins of the clan while at the same time striking a blow for equality in the vampire hunting workplace, it wasn't all that fun to play. Muddy graphics, slow mechanics, and cruel level design hampered the game more than it should have. Considering that the last Game Boy Castlevania games had come out years earlier, Konami should have been able to do a better job with the lessons learned in the interim, but instead they cranked out a game that felt too similar to those titles, if not even more archaic.
Where the game could have been a companion piece to Symphony, instead Legends ends up being a sad final note on the classic game play of old. After two disappointing sales of this game, coupled with poor U.S. reception of Symphony of the Night (at least initially), the series would strike out in a new direction. Castlevania for the Nintendo 64 would try to update the classic game play for 3D consoles (also to muddy effect and little fanfare). The failure of that game would prompt Konami to all but abandon the classic series entirely, moving forward with Metroidvania games for the foreseeable future (powered by the relatively brisk sales of Castlevania: Circle of the Moon).
As a sad final note, Sonia was later removed from the official continuity, all but ensuring her as but a footnote in the grand series history. Famously, Koji Igarashi hated Castlevania: Legends, calling it, "something of an embarrassment for the series". Sonia's role as progenitor of the series was eventually taken over by Leon Belmont in >Lament of Innocence), while her next game, the U.S. developed Castlevania Resurrection) would get canceled. Sonia was left on the sidelines, a footnote to the series instead of the grand beginning for it.