While this game practically only consists of combat, it is well designed and remains enjoyable throughout the whole game
In Chain of Memories, you play as series' protagonist Sora who is trying to find his friends after the events of the last game. He comes across the grand Castle Oblivion and, upon entering, finds out that the castle's many floors recreate scenes from his memory. Simply put, each of the thirteen floors of the castle represent each world that Sora travelled to in the original game, and they can only be accessed if you have the world's corresponding card ('World Card' as they are named in the game) which the members of an ominous organization that run the castle give to Sora throughout the story. As you can tell, trying to explain the setting of the game is quite a hard thing to do, but the important part here is that the game revolves around memories and, more importantly, cards. Rest assured, however, that everything will be clear as soon as you play the game yourself (provided you played the previous entry in the series)
The focus on cards is the important part here as every aspect of the gameplay consists of cards; and with 'every aspect of the game', I am referring to the sole mechanic that the game consists of, which is card-based combat. Do not imagine Sora sitting down at a table playing cards with the enemy heartless, however, because the game does still feature the active battle system that the series is known for.
The basics are practically the same: Run around the enemy and then hit him with an oversized key until he runs out of HP and explodes into EXP, HP and munny balls. The key difference is that every possible action is linked to a card. In order to do a slash with your keyblade you need a keyblade card, and in order to do the standard 3-hit combo, you need to use three keyblade cards one after another. Magic works the same way: Use a blizzard card, for example, to cast Blizzard.
The key here (no pun intended) is the fact that there is no block button, and that actually impacts the whole system much more than you might initially think. In order to block an attack and leave him open for an attack, you need to break the enemy's attack card by playing a card from your deck that has a higher value (0-9) than his, which is displayed on the lower half of the screen. However, you cannot just stuff your deck with a thousand key cards as you have a limited amount of card points (CP) that you can spend on making your deck as the different types of higher value cards also cost more CP to integrate into your deck, and you can only maximize your CP if you level up.
The level-up mechanic here is also different because you are given the choice of leveling up one of Sora's two stats (HP, and CP) or learning a new 'sleight' which is a special attack that can be executed by combining three cards whose values add up to a specific value. I am sure that my explanation is not good enough to convince you of the amount of depth that the combat system holds because of this focus on cards and values, but that is because I could never cover the countless aspects of the system. However, the real fun of the game comes from the fact that the enemies are bound by the same rules as you. All of the bosses have devastating special attacks, and if you do not want to get hit countless times and travel to game over screen, you need to be able to break their three-card-chains by chaining together cards of a higher value. You still have the option to dodge, and that is certainly an important part of mastering the game, but it is not easy to dodge some of the enemies attacks which is why your focus will be more on creating a deck consisting of cards that are either high in value or allow for creating the special attacks that the enemy might be weak to.
As a result, deck management is a very important part of the game but will not take as long as you might think, but still more than you would perhaps like. In order to gain more cards, you need to trade cards that you don't like with booster packs that Moogles offer you. The cards that you receive are chosen at random, but there are different tiers of booster packs that you can choose from. Again, you will simply have to trust the reviews when it comes to the depth of the battle system for it is impossible to explain all of its nuances, but believe me when I tell you that every battle, especially the boss battles, are a blast to play through because of how challenging they are. Finding the right strategy is mentally stimulating and cannot be done simply by looking up guides because of how every player has collected different cards over the course of the game (though if you are hardcore and want to completely obliterate everyone, you could grind your way to a perfect deck by collecting many cards and leveling up your CP and HP to max). It is incredible how the combat system is enough to carry the whole game by itself. It is tense, fast and, most importantly, fun.
Kingdom Hearts fans will get even more out of the game, however, as the story told in this entry of the series is very engaging as well and bridges the gap between Kingdom Hearts and Kingdom Hearts 2 in an unexpected way. It will keep you interested as it progresses at a very steady pace without ever giving away too much information.
There are still many questions to be answered (as seems to be the case with every Kingdom Hearts game) but you will be satisfied by the end of the game. It took me about 30 hours to beat and I enjoyed most of it, though the last few hours of Sora's story did get a bit tiresome as even the most engaging battle system will wear itself out at some point. Luckily the game changes up the mechanics drastically in the last 6 hours of the game (I won't say anymore than that) which makes the game just as enjoyable as it was when you first started it.
Kingdom Hearts: Chain of Memories is an incredible entry in the series, both gameplay and story-wise. The mechanics are solid and the game will keep you on your toes because of its perfect pacing and difficulty. It has turned me into a fan of the series, and I hope you will get the same kind of experience as I did. There is so much to the game that I could not mention, like the way in which you progress from room to room on each floor (which is also a mechanic linked to cards), but that is because of its simple complexity, or its complex simplicity if you prefer to put it that way. Rest assured, this is a satisfying game to play through.