Although it's unique, this game is for Kingdom Hearts fans only.
Yes, a deck. The game revolves around a card-based system, that is, while unique, is frustrating in the end. You need cards of certain levels to open doors, and that can get you stuck because you don't have a card of that level. I had to restart the game because that happened to me.
On the plus side, however, the graphics are incredible for a GBA game. Most don't expect the occasional CGI cut-scenes that appear. The character sprites look really good, and the game retains the humor from its PS2 counterparts. Character voices in battle are good, but music seems pasted in, and seems pretty strange.
A second playthrough and a multiplayer mode add replay value. But both are available only after the main story is complete. Do NOT buy this game unless you want to know what went on between Kingdom Hearts and Kingdom Hearts II or if you're prepared for the intense challenge the game offers.