The Poseidome Colosseum is the Colosseum of Atlantis. King Neptune, and SpongeBob SquarePants had a Cooking Contest in this building in the episode "Neptune's Spatula." It also appears in Battle for Bikini Bottom, where Robo-Sandy is the Boss.
It is a large concrete, Greek Building, with large concrete cylinders at the main entrance. inside there is a modern arena, with Show lights, a Ring, and many rows of viewing seats.