Moonside is unique in that it is not a real place, but is actually a psychic illusion created by the Evil Mani Mani statue. The illusion is a bizarre inversion of Fourside, bearing a vague similarity to the real city, but with the colors of streets and buildings heavily distorted, with neon-colored buildings and black streets. There is a hotel as well as a parody of the Fourside Museum.
The citizens of Moonside are incoherent at best, and invert the meaning of many words (in particular, 'Yes' and 'No'). Much of the dialogue in Moonside breaks the fourth wall, referring to game elements such as Ness and Jeff's hit points. The overall tone of Moonside, and especially its inhabitants, is that of sheer madness.
Enemies freely roam the illusion in the form of Abstract Art, Dali's Clock, Enraged Fire Plug and Robo Pump. Ness and Jeff move around Moonside with the assistance of Warp Men, who teleport the boys around the city.
At the heart of the illusion is the Evil Mani Mani statue. Once the statue is defeated in combat, the boys find themselves in a storage room in the back of Jackie's Cafe, alone save for the shattered remains of the statue and a mouse who informs them that they were wandering around the storage room in a daze. With the destruction of the statue, the illusion of Moonside ceases to exist. Despite this, any items obtained in Moonside transfer to the real world.
- A man in a suit outside the cafe says, "Wel come to moo nsi ns dem oons ide" This fits into "Welcome to Moon sins de Moonside," referencing "sins" and "demon".
- The graphical nature of the city shares a striking similarity to games that were released on the ZX Spectrum console such as Sabrewulf and Atic Atac.
- The music features heavy sampling from Ric Ocasek's Keep on Laughin'. The track itself plays for 0:20 before looping constantly.
- It's possible that the name Moonside was chosen was because the moon has four phases, referring to Fourside.