On the south side of Atlantea, you will find the Endless Dungeon. This is a level you can replay an unlimited number of times, where you can get gold and chests, level up your Hero, and generally, fight not-very-difficult AI battles. You can access this special minigame once you've conquered Skyview Tower, which is just 2 missions beyond the game's starting point (so new players will have access to this dungeon quickly).
The Endless Dungeon is notable because it’s easier to grind than the Arena, but of course, the rewards are less. For one thing, it is impossible to receive an Epic Chest after a battle in the Endless Dungeon. You can however sometimes receive Wooden, Silver, or Gold Chests. Individual battles also only award 50 coins each as opposed to the higher rates in the Arena. The one exception to this trend however is in Hero experience: Each battle in the Endless Dungeon awards 750 experience points to your Hero upon a victory. This makes the Endless Dungeon the best method to grind the Hero experience that is currently available in the game.