Fourside is a hidden stage in Super Smash Bros. Melee that is unlocked by playing 100 Vs. matches. The level is a large level that takes place on rooftops. The Monotoli Building is one of them, and its sign can be seen. On the Monotoli, the helicopter flies on and off the building. Occasionally, the UFO from the title screen of EarthBound shows up. Its surface is slippery to all characters save the Ice Climbers.
Link and Mario brawling it out on the stage Fourside.