Goblin Stadium is the first real Arena to be unlocked. You need at least 0 trophies in order to play in this arena. As it name states it focus on Goblin cards so you unlock all the cards related to Goblins in Clash Royale (Goblins, Spear Goblins, Goblin Hut and Goblin Barrel) as soon as you unlock this arena. The Goblin Stadium comes after the Training Camp and before the Bone Pit.
Goblin Stadium Cards