Arena is a mode in Plants vs. Zombies: All Stars. In this mode, the player gets to fight with other players to get into higher leagues and earn rewards. This mode also features a unique currency, medals, which is not used anywhere else.
The player will set up their own defenses to fight with other selected players' decks. Note that only plants that are Level 15 or more can be selected and the player can also "investigate" other players' decks. "Auto-play" mode can be turned on or off for the player, but locked for the enemy. Every 4 players he/she defeats, the player will get promoted to the next league and earn 4 cards of rewards, that will shuffle and the player will get to choose one as their reward. Each enemy the player has defeated will give him/her a chance to open a chest which contains rewards, or even medals. When one of the player's plants in their deck has fainted, it will be out until the player reset their progress, which can be done once a day, or pay 20 gems to revive all plants (can only be done before starting the first battle of each league). Once all plants are dead, the player can no longer fight in this mode until the progress is resetted. There is also a shop here where the player can buy Puzzle Pieces and sun with medals.