Modem Initialization Mode
At the title screen press 1 + 9. This activates the "CATBOX", the Atari Jaguars version of the X-Band that never really took off. It allowed you to link 2 or more Jags together or play via a phone line for games like Wolfenstien 3-D or Ultra Vortex.
When you reach the point of choosing your stage press '#' to access the Hidden Palace, or '*' to access the Subway Passage.
At the title screen press 1 + 5 + 9. A new option will appear under the Options menu.