The Battlesphere is 7th Stage in the upcoming sequel, Freedom Planet 2. Constructed in Downtown Shang Mu by order of Mayor Zao, the Battlesphere is a gladiator-like arena where Fighters, Racers and Heroes from all over Avalice come to put their skills to the test in a variety of challenges. Captain Kalaw is the current reigning champion.
You can unlock arena challenges in the Battlesphere by trading Star Cards here.