Nebulus

Unlimited lives and time:
Pause game play and hold [Left] + [Up] + J and press Fire.

Tower Select:
Pause game play and hold [Left] + [Up] + J + Fire and press 1 through 8 to jump to the corresponding tower.

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

Unlimited time:
Load or reset the game, then execute the following BASIC commands before running or restarting the program.
POKE 46729,0
SYS 32768