An underground labyrinth where you break through walls to advance. Judge enemy strength from the numbers on your radar, and defeat enemies at or below your own level to level up. For example, if your level is 1 and the radar also shows 1, then digging through any surrounding wall is safe. Once you're certain that "there's an enemy inside this wall," mark it with a note to make deducing easier. (With a mouse, right-click works too.)