Monster World is a peaceful realm which is home to Shion the Wonder Boy and many other humans. The name is somewhat an artifact title, as Shion rid the world of monsters in the video game Wonder Boy in Monster World. The world is much like Planet Earth, split up into towns and villages with an ocean and countryside. Shion appears to come from a small, unnamed village (although he later says he is a traveller) but this settlement is burned to the ground when Grimomen and his demon companions kidnap almost everyone in town. Only Old Kevin is left to break the news to Shion, who orders him to travel to the country of Alsedo and inform the Queen of the problem. He later returns with Alsedo's workmen to repair the village before Shion gets home.
Fisherton is a larger town on the shores of Monster World. It seemed to be a peaceful, working town before Grimomen arrived and destroyed that area too. One resident who survived the attack was a young girl called Baila. She rescued Shion after a failed attack on Grimomen and led him through Smuggler's Tunnel (home to a giant worm) before they reached the shores in secret. Baila would later visit Shion at the destroyed village and ask if he would like to return to Fisherton for a while, an offer he refuses.
Next to these two towns is an ocean. On the coast, you may be able to find poison crabs, bouncy jellyfish and annoying monkeys. The sea is no less filled with dangerous wildlife, which Shion finds when he is harassed by an unlucky octopus. The ocean also hides Lauren and Hardy, a mermaid and merman modelled after Laurel and Hardy. They point Shion to The Great Deep, an underwater jungle protected by a bubble at the bottom of the ocean. This area is presumably still part of Monster World but reminds Shion of an unvisited "Jungle World", complete with the same vicious wildlife. The forest houses a dimension portal which allows easy access between Monster World and Demon World (providing you have the right papers for Schwartz and Egger).
Another area Shion visits are the Skyrock Mountains, home to poets, philosophers and cosmologists. Shion arrives when he hears that they have been having ghost problems and soon learns it is because of the inhabitants of Ghost World have created a staircase to Monster World thanks to the Wailing Sisters of Witchery World. Shion and Lukout accidentally enter Ghost World and eventually leave with help from Wordsmith. The trio destroy the Power Jar holding the stairs in place and end all hopes of Dinotown ever getting to conquer Monster World.