The Colosseum is a location in the Second Sea that can be found in Area 1 of the Kingdom of Rose, slightly to the left of Don Swan's Mansion. The Colosseum is great for 1v1's, featuring the integrated "Friendly 1v1" system, and plays a big part in the Colosseum Quest. The Legendary Sword Dealer can also spawn in one of its arches to the north-east.
The inside looks like a circle island with it being surrounded by water. It also includes 4 mini bridges that connect to different entrances and exits. There is also a race course-like circle around the entrances, and it can, theoretically, be used as a racetrack. There is no roof on the Colosseum, similar to the First Sea's Colosseum, but unlike the first, the second Colosseum is circular, instead of oval-shaped. However, as a result, it is much smaller than the First Sea's Colosseum, probably by intention. Below under the Colosseum are gladiators, which you need to rescue to complete bartilo's quest. The code for rescuing the gladiator's is on a dinner table in swan's mansion.