This sequel adds new enemy ships to the alien fleet such as firebombers, Yllabian Space Guppies (note that Yllabian is based on "Yllab", the word "Bally" spelled backwards, a friendly poke at Williams' then-competitor, Bally Midway), Dynamos and Space Hums. The Defender ship is now equipped with an Inviso cloaking device, which renders the ship invulnerable when activated, but has a limited charge. A Stargate will transport the ship to any humanoid in trouble. There are now two special stages, the Firebomber Showdown and the Yllabian Dogfight, that occur every fifth and tenth wave. As in the first game, if all the humans are captured the planet explodes and turns all the landers into mutants.
