There's two actually, Kings Colosseum (with a Regicide component and tech buildings) and Halloween Colosseum (where you start with a limited eco).
Most use Japan for Kings Colosseum, because one of the crucial early units is a yumi or samurai, and your Post-Imperial start gives you a huge range and upgrade advantage over people playing other civs.
I've also seen people do well playing Aztec because early Maces get an important boost + you can build a firepit in your walled area and heal/boost your guys with your free villagers.
Halloween Colosseum is pretty straightforward, pick an early melee unit and go that civ. Sioux get a massive advantage early with free Dog soldiers and additional ones they can make at the Pit, then go Siege Dance and systematically dogbox or destroy opponents' eco.
Crunkatog on ESO
Bart331 balance suggestion: aztec: remove civ
Voltiguer: Ender, Sioux in 1.04 will be a top civ, no matter how many layers of Sioux goggles you put on
schildpad on Elephants: ...their mansabdar unit sucks so hard it looks like a black hole
Crunkatog on Steam.