Chao Ruins is a fighting arena in Sonic Battle. It is treated as Knuckles' home arena.
Situated on the mountainsides of Holy Summit, Chao Ruins gets its name from the ancient stone pillars that litter the arena, many of them bearing hieroglyphs that appear to resemble Chao. These stone pillars decorate the corners and the middle of the arena, and can be used for safety as well as cover from other fighters. The arena itself is moderately-sized, though its size combined with the stone pillars makes it difficult to dash away from opponents.
Throughout Story Mode, Chao Ruins is used as the battleground for all fights that happen over the course of the story until Emerl's episode. All of Knuckles' conflicts with Emerl that happen during Sonic's and Tails' episodes take place here, with the former due to Knuckles mistaking Emerl for a Phi robot, and the latter due to Emerl refusing to listen to Knuckles. In Emerl's story, Emerl can train with Knuckles here.