Castlevania: Aria of Sorrow
For many fans of the Castlevania series, Castlevania: Symphony of the Night was the high point of the series. It's heady mix of music, graphics, and game play mixed with a "new" (if we conveniently ignore Castlevania II) play style liberally borrowed from Metroid crafted something more than the sum of its parts. It was a game in the Castlevania series unlike (and, for the most part, better than) anything that had come before it.
For the years afterwards, though, Konami seemed to have problems giving fans of Symphony the follow up they wanted. Castlevania: Circle of the Moon was a huge success upon release, but was considered something of a misstep when it came to visual style and its difficulty (although, with the perspective of time, and the joys of emulating allowing us to appreciate the graphics better, that game has gone up in critical estimation). Harmony of Dissonance, meanwhile, was considered an upgrade in the visuals as they were clear and easy to see (although fans have since walked back that sentiment), and did feature the return of producer IGA, but it failed to generate the revenue Konami would have liked.
For the third Game Boy Advance title, and his third game in the series, Koji "IGA" Igarashi wanted to improve on Harmony of Dissonance while continuing to deliver the experience fans expected. Developed concurrently with Harmony of Dissonance, and released a year later, Castlevania: Aria of Sorrow (Kyassuruvania Akatsuki no Menuetto in Japan, translated as "Castlevania: Minuet of Dawn") feels like an answer to all the Metroidvania titles that came before. It has a hero, Soma Cruz, cut from the mold of Alucard, with visuals that are clearer than Circle but nicer looking than Harmony, all in a package that showed true evolution for the mechanics, and substance, of the series.
Aria of Sorrow presents the story of Soma, an exchange student (at least in the English translation) touring Japan with his friend Mina Hakuba. Set in 2035 AD (a date chosen by IGA to create a new and distinct era for the franchise), the story concerns Soma as he's mysteriously transported to Castle Dracula. He's not sure why he's there or what's going on, but he feels a dark power growing within him and, on the advice of a mysterious stranger, goes off to explore the castle to find a way out. Along the way he'll collect power-ups in the form on monster souls, and learn more about the castle, and his own place within it, in the process.
Like the previous Metroidvania entries, Soma explores the whole castle, moving back and forth, collecting power-ups and new abilities all with the goal of completing the quest and ending his tenure in the castle. What Aria managed to get right was that it was engaging. Gone were the long, sometimes repetitive hallways of Circle and the flat, muddy, listless graphics and game play of Harmony. Instead, the game was light, fast, and colorful, with an interesting mechanic (the soul collection system) to boot. It took the bones of everything that came before and synthesized it into an adventure that worked really well on the Game Boy Advance's hardware.
Over time, the legacy as Castlevania: Aria of Sorrow has grown. It's viewed by most as a high point for the Castlevania series and the point where the Metroidvania format came into its own leading to a number of great titles in the genre immediately after. Unfortunately, while its viewed fondly now, at the time of it's release it was not a huge seller for the series. Although it performed slightly better than Harmony of Dissonance, pulling in close to 160,000 units in sales in its first three months or release (and by some reports over 400,000 over the course of its life and re-releases), this came with only 27,000 of those units selling in Japan, making it a failure in its "home" market. This is what led some in Konami to remark that the franchise only appealed to Western audiences from this point forward (and likely shaded many of the decisions later in the series to boot).