Death Blow Level:
To activate the Death Blow level, go to the level selection screen and click on the spiral after you've completed Level 12.
Select any mission:
This cheat requires editing a text file. If you don't know what you're doing, we advise not messing around with this cheat.
Find the file 'shellcfg.h' in the 'darkshell' directory and change the line:
#define BTN_MISSION_COEFFICIENT 150
#define BTN_MISSION_COEFFICIENT 157
You can now select any mission from the dial by pressing the new CHEAT button in the bottom left corner.
Thanks to Revolution readers KaostiK!