After you beat the main game, you can go back and play any of the levels in any order. Additionally, each "area" has two bonus levels that are only unlocked after you complete the game. Whenever you beat one of these new bonus levels, you're taken to a screen filled with 51 (or so) cards, and one of them is flipped over so you can see what's on it. This ONLY happens when you beat a new unlocked stage, and not if you play through one of the old stages.

I've played all the unlocked stages, but that only accounts for roughly half of the cards. Are there more hidden levels you can unlock?