Coliseum is a stage appearing in Super Smash Bros. for Wii U which is based on The Arena present in most of the games in the Fire Emblem series. It is a stage of large size with no real hazard present. There is machinery underneath the stage which adds extra platforms to the otherwise empty level.
This stage appears to be based on a feature in the Fire Emblem series, which is known as The Arena, rather than Coliseum (Coliseum is most likely what it's called in the Japanese versions). Arenas appear as small buildings on the map. You can send a character there to fight for a gold reward, but if that character loses they die.