Nightopia is half of the dreamworld in the NiGHTS franchise. This is the side of the dreamworld where NiGHTS and the Nightopians are located in. Here's where visitors go in their dreams. Wizeman and the other Nightmarens want to take total control of Nightopia because they want total control of the dreamworld, spreading fear and hatred throughout the dreamworld.
Nightopia shapes it self based on the visitor who enters it. Usually, visitors can't go to other visitor's Nightopias, but it sometimes happens. Such as with Will and Helen.