Twilight, simply put, loves Duel Monsters. Just the strategy of it that actually makes her think is what kindled her enjoyment for it. Watch Twilight play a children's card game against a "holographic" (holographic meaning it can touch cards, plays cards, etc. for the entire duration of the duel, then it disappears) AI she successfully made appear using magic.

Just a simple card game, is what it is.

Also: If anyone can find a good cover picture of Twilight playing Duel Monsters, it'd be greatly appreciated. Seriously, Google Images is no help right now.