In the Simon Wai prototype of Sonic the Hedgehog 2, selecting "Genocide City Zone" from the Level Select screen will bring the player to an empty level and then simply fall to their death at the bottom of the stage. Concept art was revealed, showing Sonic in a traveling tube, along with the name Cyber City Zone.
Tom Payne, the zone artist in charge of this level, stated that this Zone was intended to be a graphical change of Metropolis Zone, but was dropped before it was even started. Instead, he worked on the third act of Metropolis Zone. According to a Tom Payne interview, the meaning behind the name is due to the Japanese team members wanting to make the zone sound dangerous in English, resulting in the alternate name "Cyber City" in concept. It was planned to be a single zone, but due to time constraints, the remaining zone ended up as Metropolis Zone Act 3. In the Sonic Jam strategy guide Yuji Naka stated basically that the third act of Metropolis Zone used the layout meant for Genocide City and that it was meant to be a single act. The concept art was also confirmed as being the inspiration for the Sonic the Hedgehog Spinball level, The Machine.