There are several different revisions of the game with small but notable changes.
• The earliest versions, released in 1999, required the disc to be in the drive, but the 2001 release removed this requirement.
• The trophies on the player's license for beating each boss go unused in the 2001 release.
• The map on the loading screen in the original release filled the entire screen, and these were gradually changed in subsequent versions, with the map being reduced in size and displayed in the middle of the blue studded background used for menu screens.