Press CTRL+F1 on the main screen. Enter the codes EXACTLY as written.

Half Libel
Had A Nude On
Brass Clue
Rotted Drop
Horny Elk Leer
Dunk It Here
Oh No! Less Japan

In the game press:
CTRL+I for invincibility
CTRL+Z for ammo ( except towers )
CTRL+F target anybody and press it. It destroys them immediately
CTRL+B Teleports next to any target
CTRL+P Long range radar
CTRL+X gives you a weapon