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Reminder: the nightmare difficulty level was discovered.
To set it make sure that you have the default game settings:
Music:on Sound:on Monsters are:normal Smooth scrolling:on Floating cursor:off
and save the game. Now open it with hex-editor, find the symbol 1B at position 00000253 (disk version) or 0000025E (CD version), and replace it with 3B. Load the game and check the game controls. If you see the
Monsters are:Main menu
setting you're in the nightmare now. To get back just click the button.

This is more brief and accurate version of the algorithm than the previous one.