After the success of Symphony of the Night, one thing many fans of the series were clamoring for was "more, more, more!" It seemed inevitable that a sequel of some kind would come out that capitalized on the Metroid-style mechanics Symphony (re)introduced into the series. And although it took four years for Konami to release their next Metroidvania, fans got what they asked for in Circle of the Moon.
Set in a Carmilla's castle (not Dracula's), Circle is concerned with the evil vampire sorceress and her plan to bring Dracula back. To combat the forces of evil, we are introduced to new heroes in the series, the Baldwin and Graves clans. Protector's of the sacred whip, the Vampire Killer, these heroes head to the castle to try and defeat Dracula before he's able to full resurrect. Things don't go to plan and it's up to Nathan Graves, current holder of the whip, to explore the vast castle and eventually reach the Dark Prince.
Although many (including the editors of this site) praised Circle of the Moon when it came out, the game was not without it's detractors. The graphics sometimes appeared a bit muddy in the game, without the smooth, flowing graphics the Playstation was able to create (the Game Boy Advance, while a good handheld, was never as powerful as Sony's set-top console). And while the castle was vast, the game didn't play quite like Symphony -- as a whip-wielder, you're limited in your weapon selection (i.e., one weapon, the whip) and the random magic you could find.
Plus, for many, the game was a disappointment simply because Alucard wasn't in it. Some fans really like the son of Dracula.
Detractors or not, Circle of the Moon was the next step in the series's evolution from action platformers to exploration heavy adventure games. Whatever it's place in fan's hearts, it deserves a strong footnote for showing the series could continue to explore the Castleroid setting... which it did, for many more games to come.