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