For a gameboy game, this is great! Following directly after the ending of the first Kingdom Hearts, Sora and friends ends up at the Oblivion Castle. Using mainly a combination of different cards that you put together in a deck, you'll then use those cards to attack. Each card has it's own number and generally the higher the number the harder is it for an enemy to break your attack. This new battle system proves to be addicting and fun, but at the same time, repetitive. Althought the story wise and the structure of each world is done greatly in this 2D gameboy world. Most of the battles proves itself to be quite annoying especially when almost every battle is to carry out the same routine of using your cards the same way as the last and further improving it to make boss battles easier.
Not only the battles, but each of the world is almost a direct copy of the previous one. Each world is made up of various number of room cubes where each room in every world is shaped like a cube. The only difference is in different worlds, the background theme is different.
At either rate, this game is the link that chains both Kingdom Hearts I and II together, so if you're planning to get Kingdom Hearts II, you should get through with this game first.