Naka's relationship with Sega was tenuous during this time, and he received little credit for his work. He left the company shortly after the game's release, although Sega of America hired him later. Before leaving, however, he defied Sega's prohibition of developer credits by displaying a few names in black text on a black background, identifiable only by looking at the code. Naka stated that level design was a major challenge: he created maps much wider than normal and tried to ensure players would not get lost. It took him around eight months to develop Green Hill Zone as he kept restarting from scratch. He stated that he found the process "very interesting". Naka also stated that the team was trying to create smooth maps, and that implementing looping structures was a challenge because Sonic would break through them instead of running around them. The backgrounds were also a challenge, as the game's speed created the impression of going backwards. The zones were based on designs by Naka and Ohshima, with the goal of creating the world's fastest action game. According to Ohshima, Robotnik was based on Humpty Dumpty.