Nemesis

Invincibility:
Press [Shift/Lock] press [Space] five to six times.
The letter "c" will appear in the top left corner to confirm correct code entry.

Invincibility (alternate):
Hold [Shift] and press [Space] four times at the title screen.

Invincibility (alternate):
Load or reset the game, then execute the following BASIC command before running or restarting the program.
POKE 10239,166

Unlimited lives:
Load or reset the game, then execute the following BASIC commands before running or restarting the program.
POKE 5868,255
SYS 5768

Unlimited lives (alternate):
Load or reset the game, then execute the following BASIC commands before running or restarting the program.
POKE 5975,234
POKE 5976,234
POKE 5977,234
SYS 5779

Cheat mode:
Load or reset the game, then execute the following BASIC commands before running or restarting the program.
POKE 32979,181
SYS 32768