At its launch in 2001, Game Boy Advance was a system about to step into the future of portable gaming, while simultaneously attempting to push down hardware costs whenever possible. The first version of the console boasted a full-color screen and 32-bit CPU, but Nintendo disappointingly omitted any sort of screen lighting solution, resulting in infamously poor visibility without a third-party light accessory. Unlike the SNES, there is no dedicated audio processor on the GBA's mainboard, forcing developers to use the system's primary CPU (already quite busy handling graphics and game logic) for any sound in games. Considering these points, one could certainly make the argument that the Game Boy Advance was a fundamentally compromised gaming system in multiple ways.
In the case of Castlevania: Circle of the Moon, the software reflects its host platform with a number of flaws. On original hardware, the first thing you would likely notice is how incredibly dark the graphics are; while appropriate for this series, this aesthetic choice doesn't lend itself well to the GBA's already murky screen. Even when playing on modern hardware, this is a dingy-looking game, and there are plenty of small enemies and projectiles that can often be difficult to pick out against the background artwork.
The gameplay also feels like a regression compared to its immediate predecessor, Symphony of the Night. In contrast to the nimble dhampir Alucard, our new whip-wielding protagonist Nathan Graves hews closer to the classic NES-era Castlevania heroes. In other words, he's extremely slow; maybe even slower than Simon or Trevor Belmont. Fortunately, the game almost immediately provides a speed boost in the form of the Dash Boots item, but you have to double-tap-hold a direction to use it, resulting in a lot of extra D-pad inputs just to avoid feeling like Nathan is stuck in molasses.
Another gameplay issue comes up in the overall design of Dracula's castle, which feels like a shoddy imitation of Symphony's. I lost count of the number of rooms consisting of a long, straight corridor filled with repeating enemies, and a single stat-boost item sitting at a dead-end. Circle is also a remarkably more difficult experience than Symphony, especially as you approach the endgame. There's a particularly nasty sewer section full of ghosts that constantly attack you with ice magic, and without a certain combination of damage-negating cards, it's exceedingly difficult to survive between save points.
Speaking of cards, the game's Dual Set-up System (DSS) attempts to make things more interesting by introducing special cards that sometimes drop from defeated enemies. There are two sets of cards: one set based on Roman gods, the other based on mythological creatures. Players can create a variety of beneficial effects by pairing one card from each set in different combinations; for example, pairing the Mercury and Salamander cards allows Nathan to attack enemies with a fire-enhanced whip. While DSS does give you a reason to grind aside from gathering experience points, the drop rate of the cards is quite low, and there are only a few critical combinations that make a significant impact on the overall difficulty.
One bright spot is the game's music; Konami clearly wanted to showcase some nice tunes for Circle, so presumably the game was programmed in such a way as to allow for more CPU cycles to be dedicated to audio. However, this game leans on recycled music from older Castlevania titles a bit more than some other entries, but at least it all sounds pretty good.
I did enjoy some of my time with Circle of the Moon, but by the end, I was finishing it more out of spite than anything else. Unlike later entries such as Order of Ecclesia, this game isn't difficult in a way that makes me want to get better at it. If Konami Kobe can't be bothered to learn any lessons from what did and didn't work about Symphony of the Night, why should I be bothered to slog through a sub-standard Castlevania game?