The Poseidome Colosseum is the Colosseum of Atlantis.
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.
King Neptune and SpongeBob SquarePants had a cooking contest in this building in the episode "Neptune's Spatula."
Battle for Bikini Bottom
It also appears in Battle for Bikini Bottom, where Robo-Sandy is the boss, and the player must defeat her.
It is one of the arenas which appears in Nicktoons MLB.