The plot is basically a re-worked version of Symphony's, but instead of the father-son dynamic this game has a more prominent rivalry component. The dialogue isn't as artful or melodramatic as said release, and reduces the hero, Nathan, to mostly uttering one-word exclamations. Descendants of the Belmont clan, Nathan Graves and Hugh Baldwin, have been trained by the latter's father, Morris Baldwin, to become vampire hunters. 10 years prior, Morris and Nathan's parents had destroyed Dracula, the Graveses dying in the process. Now, in 1830, Morris has entrusted Nathan with the Vampire Hunter whip, much to Hugh's chagrin. When the vampire Camilla resurrects the Prince of Darkness once more, the three enter the castle, but are too late, as Morris is captured and his two apprentices are sent down to the Catacombs. There they split up, Nathan intent on defeating Dracula and rescuing his master and Hugh intent on going it alone and proving himself greater than his rival. When the series was given back to Koji Igarashi, who created it exclusively for almost a decade beginning with Harmony of Dissonance, he controversially removed Circle of the Moon's story from the overall timeline.
Nathan Graves operates more like a Belmont than Alucard, with the same weight and rigid movement. He can use: 'A' to jump and again to double-jump when he finds the appropriate power-up; 'B' to whip and, when held, to automatically spin it around in a circle, without requiring the player to manually brandish it as in Super Castlevania IV; 'Up' and then 'B' to use a sub-weapon (the traditional Dagger, Holy Water, Boomerang, Axe, and Stopwatch - the Holy Water being particularly effective compared to its previous series iterations); 'Left' or 'Right' twice on the D-Pad to dash, once that power-up is collected; 'Down' and then 'A' to perform an offensive slide on the ground; 'L' to turn on DSS magic; 'R' in conjunction with other buttons to perform special abilities that must also be won first; 'Start' to view the pause menu; and 'Select' to view the map. As stated, some of these abilities must be earned first as Magic Items, though the amount of actually useful ones versus the amount of ones that simply remove an obstruction are split down the middle. The Dash Boots, Double, Kick Boots, and Roc Wing are useful in their own ways, but the Tackle, Heavy Ring, Cleansing, and Last Key are hardly used for anything beyond making passage possible or, in Cleansing's case, significantly easier.
The RPG system has been greatly simplified. Rather than two qualities each that affect damage dealt and damage taken, there are simply STR (Strength) and DEF (Defense), along with INT (Intelligence, which affects MP recovery rate) and LCK (Luck, which affects drop rate of items) - the latter two having no direct effect on combat in this case. Equip armor and stat-altering items are also in a smaller amount (for instance, 5 HP-restoring items compared to the dozens in Symphony) and fewer categories (Body, Left Arm, and Right Arm for armor and Use for expendable items). HP, MP, and Hearts Max Increases are to be found throughout the castle, HP and MP maxes increasing by 10 points and one's Hearts max increases by 2 hearts. Experience gaining in Circle of the Moon is less natural than it was in its predecessor, and thus there's more and longer grinding involved, but the amount that enemies give the player doesn't diminish over time, and thus the highest level of 99 is quite achievable. Even in this state, the enemies offer a veritable challenge without DSS involved, and though the game's nothing a seasoned conqueror of the NES games will sweat over, it's closer than Symphony of the Night to being true to the series' reputation for difficulty.
The Dual Set-up System, or DSS, involves the player collecting 20 different magical cards from enemies, 10 Action cards depicting Roman gods and 10 Attribute cards depicting mythological monsters. The system is based around selecting one card of either type, the Action card deciding the type of effect it will have and the Attribute card deciding what elemental attribute will go along with it. By collecting every card, the player will unlock all 100 spells in the game, a truly formidable selection even when compared to those that would come later. Of the spells, there are elemental whips and alternate weapons, stat alterations, protective barriers, sidekick-esque familiars, devastating summons, and others. Pretty much every Belmont ability from games prior to Circle are included in some shape or form, thanks to DSS.
There's just one map in the game, and it features plenty of unconventionally-shaped rooms that don't adhere to the standard square or rectangular form that Iga would stick to in his games. The HP, MP, and Hearts power-ups are generally either hidden in the abundant breakable walls leading to secret rooms or hallways set completely to the side of the main path, instead of their placement feeling naturally apart of the rest of the castle. The game's progression feels similarly stilted, with a whole, isolated section of the castle being blocked off by one obstacle with one escape route. The Battle Colosseum is a nice side-quest to the main game: Nathan has to defeat all the enemies in a horizontal, rectangular room before advancing to the next one, and must continue slaughter-after-slaughter - with breaks for heart-replenishing and escape - until he reaches the end, where he'll be rewarded with the Shining Armor, the best armor in the whole game. Completing this is entirely optional, and yet it's a novel concept and a likely inspiration for Harmony of Dissonance's Boss Rush Mode, which would become a staple feature of every Iga Metroidvania afterward. As in Super Metroid, the developers - either intentionally or accidentally - left room to skip ahead. As a result, it's possible to ignore the Underground Gallery and Underground Warehouse and go straight to the Underground Waterway after defeating Adramelech; however, the uncleansed water will still be poisonous and the enemies will be stronger than the player might be prepared for. While this was either a conscious decision or an unintended one, both speedrunners with a mission and totally unaware players have skipped these large portions of the game.
There are several extra modes to be played upon normal completion of the game, or Vampire Killer Mode. The first is Magician Mode, in which Nathan has greater Intelligence and MP and boosts these stats faster than normal upon leveling up, as well as having every DSS card in his possession from the start; Fighter Mode, in which he has greater Strength, Defense, and HP - stats which go up more quickly than in other modes - and can level up faster; Shooter Mode, which makes sub-weapons much stronger and gives Nathan a much greater amount of Hearts that he can increase even more by leveling up, as well as the Homing Dagger sub-weapon, being able to seek out enemies that aren't directly in his line of fire; and, finally, Thief Mode, which starts him out with extremely high Luck and greater potential to increase it - with all of these modes, stats that aren't improved are generally lowered. Also, by combining the Black Dog and Pluto cards and turning on DSS Nathan will transform into a skeleton that can throw bones, occasionally an incredibly large and powerful one, but that is killed in one hit. By equipping the Bear Ring before using this spell Nathan will turn into the BearTank character from Rakuga Kids for the Nintendo 64, which can shoot missiles and lay bombs but is also KO'd by one enemy attack. It might have been a better use of these characters if they were featured - without the same vulnerability - in extra modes to the Nathan campaign, along with an alternate mode in which the player could control Hugh Grant. Playing with alternate stats offers some variability, but not as much as well-balanced extra characters.
The graphics are a great aspect of the game, but hindered it due to the hardware of the time. The castle is truly dark and gritty, and enemy sprites are menacing; but since the original Game Boy Advance had no backlighting, it became an issue. Nowadays, it's easier to appreciate them, though Nathan's and the rest of the humanoid primary cast's sprites are rather bland and, barring Camilla's, don't have much personality to them. The is likely due to their small size, which limits the pixel amount that could be spent on them, though this doesn't seem to hold back Dracula's lesser minions. The music, by Takashi Yoshia, is up to the series' high standards, though the majority of the score is made up of rearranged versions of songs by other series composers; the re-workings are of high quality, however, and the new tracks blend into the tone set by the other ones. Of the original music, there are: 'Awake', playing in the Catacombs; 'Fate to Despair', heard in the Eternal Corridor; 'Proof of Blood', going with Nathan's final battle with Dracula; and 'Repose of Souls', which ends the game on a soft and nostalgic note. Of the redone songs, there are: 'Vampire Killer' from the original NES Castlevania; 'Clockwork', 'Aquarius', and 'Nightmare' from Castlevania III: Dracula's Curse; 'Clockwork Mansion' from Super Castlevania IV; 'A Vision of Dark Secrets' and 'Dance of Illusions' from Rondo of Blood; 'Sinking Old Sanctuary' from Bloodlines; and 'Sign of Blood Pulse' and 'Shudder' from Castlevania for the Nintendo 64. The Gregorian chant title and save screen track isn't even a new version, but an exact copy of Rondo of Blood's 'Requiem'. While the graphics didn't complement the original screens they were viewed on and the music is borrowed for the most part, they create the classic series atmosphere effectively and are surely strong points.
If scrapping most of the intricacy of Castlevania: Symphony of the Night, the handheld iterations that followed would provide interesting alternative magic systems, and Circle of the Moon's DSS is one of the most engrossing. The colosseum is a fun side-adventure as well, and the game's challenge is more in line with what's expected from the series. Though atmospheric, its dark colors made it hard to look at on the non-backlit Game Boy Advance, and it's overall not as complete an attempt at the Metroidvania formula as it could have been. Being the first of its kind on a sixth generation handheld system, it's definitely an impressive effort. The next year, Koji Igarashi and his team would be back to recreate Symphony of the Night - and then again, and then again.