The first excursion of Castlevania for the Game Boy Advance is an amazing game, period.
Circle of the Moon is a side-scrolling action-adventure game with RPG elements. Using your infamous Vampire-killing whip, you fight through countless monsters and some darn tough bosses, leveling your character up and obtaining items along the way.
What makes Circle of the Moon stand out, is the unique DSS card system. Certain monsters contain cards that you can combine with another card, and you will be rewarded with a special perk with the two equipped. Unfortunately, the system IS glitchy. If you don't feel like collecting all of the cards, theres no need, as you can use their powers without them.
The game isn't necessarily long ( a little shy of ten hours ), you'll play it over and over again, with unlockable modes, not to mention how much you'll be dying and restarting.
The difficulty IS hard, if you decide to play the game fairly. There are alot of cheap ways to take care of tough enemies/bosses, which is good if you are stuck, but mostly is just disappointing. Leveling is easy, it seems to go relatively quickly, and its fun. The game is good about making sure you are at the level you should be without alot of grinding, which is nice.
Castlevania: Circle of the Moon, made its way to becoming one of my favorite games, despite some minor glitches and cheap ways out, and is recommended to anyone looking for a good adventure.