This is my all time favourite Castlevania game ever. It plays in a similar vein to Symphony of the Night, so if you liked that Playstation title then you will surely like this one.
The graphics are serviceable. Meaning, it is as good as you can get on the GBA. My only problem with the graphics is that some of the backgrounds around the castle are dark, so some enemies tend to blend in with the background and get cheap shots in.
The control scheme and moves list are perfect. And there really is a lot to do in this game. It has RPG elements so you may find that you may have to trudge through the castle for some more time in order to level up before fighting the bosses and visiting certain parts of the castle, as those enemies will make quick work of you. Aside from the special abilities that you can gain that gives you access to different parts of the castle there is a magic (I can't remember what it is called but it drains MP, which I assume stands for magical power) system that can give you certain abilities like a fire whip or a protective barrier. It is a card combination system, and different combinations give you different abilities. I am yet to discover all the abilities in the game because these cards seem to be dropped at random by different enemies that you kill, and there are just too many combinations to discover. If you like good looking, engrossing gameplay then this title is surely for you.
Lastly, the sound in this game is top notch. From the haunting music in the title screen to the in game music and sound effects Konami did a spectacular job with the sound in this game. The only problem with the sound design is that the character yells every time he jumps or swings his whip, so you'll be hearing that sound effect a lot.
In terms of difficulty, Circle of the Moon is a pretty difficult game, but it becomes more bearable when you pick up the strategy of just trudging through the castle and killing wave after wave of enemies to level up because as mentioned earlier if you try to take on the bosses and certain enemy characters and your character is not leveled up enough then you won't last long. Also, you can gain stat boosts by equipping certain items like different weapons and articles of clothing and jewelry that you can pick up. This game is truly a gem, and aside from the dark backgrounds that make things hard to see on the screen at times I don't think that there is anything wrong with this game at all.