Yu-Gi-Oh! The Eternal Duelist Soul is a Yu-Gi-Oh! video game for the Game Boy Advance. It is the international version of Yu-Gi-Oh! Duel Monsters 5 Expert 1. Gameplay is similar to standard dueling rules, except that the Side Deck allows less than 15 cards, and is more for convenience of getting to cards for your Deck than switching between duels, as CPU duels are single duels rather than matches (with some exceptions in events).
You begin with either a red, green, or black deck. Opponents are arranged in “tiers,” with higher tiers unlocked from already open tiers. A tier may consist of 4 or 5 duelists. In order to advance, all duelists on a tier must be beaten a certain amount of times before the next tier can be unlocked. Beating a tier also unlocks certain in-game events, such as special cards that can only be received by beating that tier.
Another important aspect of the game is its in-game calendar. On certain days of the week (not actual days, as the game does not support a real date function) in game, random events may occur, such as receiving a pack of cards in the mail or being challenged to a Match by an opponent. Each time after finishing a duel, the calendar moves on to the next day.
This is also the first game to include Polymerization as an actual card.